Toxicology Principles and Exposure Testing

Every substance is toxic at a sufficient dose, and the dose-response curve describes that relationship. Cumulative response frequency plotted against dose defines ED₅₀, the dose effective in half of a population, TD₅₀, the dose producing toxicity in half, and LD₅₀, the dose lethal to half. Human LD₅₀ values are extrapolated from animal studies and are never measured directly. Wide separation between ED₅₀ and TD₅₀ means a larger usable margin, which is the therapeutic index covered in the therapeutic drug monitoring lesson.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Acute and chronic exposures require different specimens. Acute toxicity follows a single sufficient exposure and is usually assessed by a concentration in blood. Chronic toxicity follows repeated exposures that are individually below threshold, accumulates over months to years, may injure organs that an acute exposure spares, and is usually assessed in a specimen that reflects accumulated burden, most often urine.

Toxicology testing has two tiers. A screen is presumptive: an immunoassay reports that something in the specimen bound a class-directed antibody above a threshold. It does not name a compound. Definitive identification requires a method that separates and identifies by structure or mass, in current practice gas chromatography with mass spectrometry or liquid chromatography with tandem mass spectrometry. A second immunoassay does not confirm the first, because both depend on antibody cross-reactivity. Report a presumptive result with the method and the cutoff stated. Investigate a discordant pair, and recollect when that is still possible.^{22 Clinical and Laboratory Standards Institute. Toxicology and Drug Testing in the Medical Laboratory. 2021.}

Exposure analytes follow the same two-tier approach without an immunoassay in the first tier. An elevated total urine arsenic is a screening result that speciation either confirms or attributes to nontoxic seafood arsenicals. A carboxyhemoglobin fraction from a co-oximeter is a clinical result that gas chromatography can confirm. An enzymatic ethanol result is limited in the same way, because it does not detect methanol or ethylene glycol.

The limit of blank describes the highest result a blank is likely to produce. The limit of detection is the lowest concentration reliably distinguished from a blank. The limit of quantitation is the lowest concentration meeting stated precision and bias goals for a numerical result.^{33 Clinical and Laboratory Standards Institute. Evaluation of Detection Capability for Clinical Laboratory… 2012.}

A cutoff is the threshold used to classify a result as positive or negative. It serves the test's clinical or administrative purpose. It does not define detection or quantitation capability. A negative result may contain analyte below the cutoff. Screening and definitive methods can use different cutoffs and detect different drugs or metabolites. When a positive screen is not confirmed, compare the compounds covered, cutoffs, cross-reactivity, and specimen identity before assigning a cause. A targeted mass-spectrometry panel cannot confirm a compound it does not measure.^{44 Snozek CLH, et al. ADLM guidance document on laboratory testing for drugs of misuse… 2026.}

A panel reports results only for the analytes it includes. If the suspected substance is absent from that list, the panel provides no evidence about its presence or absence. Naming the suspected agent on the order lets the laboratory choose a specimen and a method that can detect it.^{44 Snozek CLH, et al. ADLM guidance document on laboratory testing for drugs of misuse… 2026.} The drug-by-drug cross-reactivity of the immunoassay panel, its detection windows, and specimen validity testing are covered in the therapeutic drug monitoring lesson.

Clinical and regulated testing are different activities with different documentation. For patient care the laboratory selects its own panel and cutoffs, and chain of custody is not required. Regulated workplace testing uses published cutoffs, a certified laboratory, a medical review officer, a split specimen, and unbroken custody documentation.^{55 Substance Abuse and Mental Health Services…, et al. Mandatory guidelines for federal workplace drug testing programs. 2025.}

Chain of custody is the set of procedures documenting the integrity of each specimen and aliquot from the point of collection to final disposition. The custody document names and is signed by every individual who handled the specimen or an aliquot, with the date and the purpose of that handling, and it accompanies donor identification, a tamper-evident seal initialed by the donor, and secure storage.^{55 Substance Abuse and Mental Health Services…, et al. Mandatory guidelines for federal workplace drug testing programs. 2025.} When a result may be used in an employment or legal proceeding, chain-of-custody procedures must begin at collection. A routine clinical result cannot acquire that status later.^{22 Clinical and Laboratory Standards Institute. Toxicology and Drug Testing in the Medical Laboratory. 2021.}

Postmortem specimens follow different rules. Drug and toxin concentrations redistribute after death, so peripheral blood from a femoral vessel is preferred over central blood, and vitreous humor (the gel inside the eye) and liver are alternative specimens. The sampling site is recorded with the result, because the concentration depends on the site.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Contamination is the main source of error in trace-level testing. Clothing, skin, the collection environment, and the container and its preservative all contribute, so certified contaminant-free supplies are required for metals. The trace elements lesson covers those collection requirements.

Specimen choice follows how the compound is absorbed, distributed, and eliminated. For several of the analytes below, the wrong specimen can give a normal result during a real exposure.

Parent alcohols cause central nervous system depression, but their metabolites determine much of the later toxicity and most of the laboratory findings. Ethanol forms acetaldehyde then acetate; methanol forms formaldehyde then formate; ethylene glycol forms glycolate and oxalate; isopropanol forms acetone and no acid metabolite.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Formate produces the high-anion-gap acidosis and the retinal and optic-nerve injury of methanol poisoning. Glycolate produces the acidosis of ethylene glycol poisoning, and oxalate precipitates as calcium oxalate in renal tubules. Isopropanol produces prolonged ethanol-like depression without severe acidosis, because acetone is not an acid. Ketosis without acidosis or hyperglycemia suggests isopropanol ingestion.

Ethanol methods and units. Enzymatic assays couple alcohol dehydrogenase to NAD and monitor NADH generation at 340 nm. They are largely specific for ethanol and do not measure methanol or ethylene glycol. n-Propanol is the main cross-reactant, and the size of its response differs between methods, so the method insert states it. Headspace gas chromatography with flame ionization detection is the medicolegal reference method and resolves ethanol, methanol, isopropanol, acetone, and other volatiles in one run.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.} A blood alcohol concentration of 80 mg/dL, printed as 0.08% in some report formats, is the common per-se limit for adult noncommercial drivers in the United States, and limits vary by jurisdiction and driver category. Serum and plasma contain more water than whole blood, so a serum or plasma ethanol runs about 1.10 to 1.18 times the whole-blood value that a statute names, averaging near 1.15. Report the specimen type with the result, because a clinical serum ethanol is not interchangeable with a statutory blood alcohol concentration.

Medicolegal specimens go into a tube containing sodium fluoride and potassium oxalate, filled to the fill line without overfilling, and refrigerated. Fluoride prevents both glycolysis and in vitro fermentation of glucose to ethanol by contaminating yeast. A large headspace increases ethanol loss during storage, which is why the fill volume is specified. Site antisepsis uses a non-alcohol agent such as povidone-iodine, because an isopropanol-containing or ethanol-containing swab adds alcohol at the puncture site.

The osmolal gap. An unmeasured low-molecular-weight solute raises measured osmolality above the calculated value, and the parent alcohols are such solutes. The water-balance lesson covers the calculation. Each divisor below converts a concentration in mg/dL to an osmolal contribution and is the molecular weight divided by 10, which assumes ideal osmotic behavior.

Ethanol does not behave ideally. An empirically derived equation published for it computes the ethanol contribution as the concentration in mg/dL divided by 3.7, with a small negative intercept, where the theoretical divisor from ethanol's molecular weight would be 4.6.^{66 Purssell RA, et al. Derivation and validation of a formula to calculate the… 2001.} A divisor of 3.7 assigns a larger osmolal contribution to the same ethanol concentration and therefore leaves a smaller residual gap. Apply the coefficient your laboratory has validated, and state it in the report comment.

Two analytical requirements apply to any osmolal gap used this way. First, the osmolality must be measured by freezing point depression, because vapor pressure osmometry loses volatile alcohols to the vapor phase and gives an osmolality that does not include the alcohol.^{77 Walker JA, et al. The missing gap. 1986.} Second, no normal gap is universally valid, because the value depends on which calculation formula the laboratory uses. One review counted 36 published formulas.^{88 Kraut JA, et al. Toxic alcohol ingestions. 2008.} The familiar range of negative 10 to positive 10 mOsm/kg is a practical convention and has not been validated as a reference interval. In one study of healthy blood donors, the median gap was negative 1, with observed values from negative 13 to positive 8 and an outlier-corrected interval of negative 6 to positive 5. Its authors suggested a reference value of 5 mOsm/kg or below for healthy adults and, from their medical inpatient data, kept a gap of 20 mOsm/kg or above as the point that raises suspicion of toxic alcohol ingestion.^{99 Skaaland H, et al. Reference values for osmolal gap in healthy subjects and in… 2020.}

The two gaps move in opposite directions over the course of a toxic alcohol poisoning. Early after ingestion, the parent alcohol increases the osmolal gap, and the anion gap and bicarbonate stay near normal. As the alcohol is metabolized, the osmolal gap falls and the anion-gap acidosis rises. A late presentation can therefore show a normal osmolal gap with severe high-anion-gap metabolic acidosis, because the parent alcohol has been converted to its acid metabolites. Neither a normal osmolal gap nor a normal anion gap at a single point excludes a toxic alcohol.^{1010 Woo MY, et al. Artifactual elevation of lactate in ethylene glycol poisoning. 2003.} The anion gap lesson covers the calculation and its other causes.

Two interferences specific to ethylene glycol. Enzymatic ethylene glycol assays use glycerol dehydrogenase and cross-react with propylene glycol, the solvent in several intravenous drug formulations including lorazepam, and with 2,3-butanediol. Reaction-rate kinetics on the analyzer flags much of that interference, and a positive enzymatic result in a patient receiving propylene glycol needs gas chromatography before you report it as ethylene glycol.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.} Separately, glycolate cross-reacts on lactate oxidase electrodes, so a blood gas or point-of-care lactate can read far higher than a laboratory lactate measured by a different chemistry on the same specimen. The difference between the two results is called the lactate gap, and it suggests ethylene glycol.^{1010 Woo MY, et al. Artifactual elevation of lactate in ethylene glycol poisoning. 2003.}

Carbon monoxide is a colorless, odorless combustion byproduct that binds hemoglobin with an affinity 230 to 270 times that of oxygen. It forms carboxyhemoglobin, which reduces oxygen-carrying capacity, and it left-shifts the dissociation curve at the remaining sites so that the oxygen still bound is released less readily. Carbon monoxide also binds myoglobin, cytochrome c oxidase, and other heme proteins, so impaired oxygen delivery is only part of the mechanism. The acid-base lesson covers the curve and the co-oximetry channel that reports these fractions.^{1,11,121 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.11 Centers for Disease Control and Prevention. Clinical guidance for carbon monoxide poisoning. 2026.12 Agency for Toxic Substances and Disease… Interaction profile for carbon monoxide, formaldehyde, methylene… 2026.}

Endogenous heme catabolism produces a small carboxyhemoglobin fraction, and one representative nonsmoker whole-blood reference interval runs to about 2%. Catalog interpretive ranges differ: one extends to roughly 5% at one to two packs of cigarettes a day and roughly 9% at three to four, another states up to 3.5% for nonsmokers and up to 8% for smokers, so interpret the result with the smoking history and the performing laboratory's own intervals.^{1313 ARUP Laboratories. Carboxyhemoglobin quantitation, whole blood by co-oximetry…} The Centers for Disease Control and Prevention states that an elevated carboxyhemoglobin level, 2% in nonsmokers or above 9% in smokers, strongly supports a diagnosis of poisoning. The fraction indicates exposure but does not grade it or predict severity, outcome, or response to treatment. Older tables that pair a percentage with symptoms are unreliable. Collection timing matters as well, because the fraction falls once the patient is breathing room air, and a specimen collected several hours after removal from the source may be uninformative.^{1111 Centers for Disease Control and Prevention. Clinical guidance for carbon monoxide poisoning. 2026.}

Multiwavelength co-oximetry is the routine method, and gas chromatography is the reference method for low fractions. The specimen is whole blood, venous or arterial, collected with heparin and kept anaerobic with the cap in place, because room air alters the fractions. At least one reference laboratory rejects EDTA specimens outright, and skin puncture is unacceptable. A blood gas co-oximeter analyzes a heparinized syringe within minutes. A send-out carboxyhemoglobin has its own refrigeration and stability window, so follow the performing laboratory's requirements.^{1313 ARUP Laboratories. Carboxyhemoglobin quantitation, whole blood by co-oximetry…} Documented interferences include fetal hemoglobin, bilirubin, lipemia, methylene blue, and sulfhemoglobin, and their direction and size depend on the analyzer. Standard two-wavelength pulse oximetry cannot separate carboxyhemoglobin from oxyhemoglobin and reads normal, so a suspected exposure requires a measured blood fraction. A fingertip pulse CO-oximeter is a different device with its own validated performance.^{1414 Agency for Toxic Substances and Disease… Nitrate and nitrite toxicity.}

Methemoglobin is hemoglobin whose iron has been oxidized to Fe³⁺, which cannot carry oxygen. The ferrous subunits alongside it bind their oxygen more tightly and release it less readily, so the defect is functional: total hemoglobin does not fall. Cytochrome b5 reductase, the NADH-dependent enzyme also called methemoglobin reductase, normally keeps the fraction below about 1%, and laboratory reference intervals are commonly stated as under 2%.^{1313 ARUP Laboratories. Carboxyhemoglobin quantitation, whole blood by co-oximetry…} Congenital methemoglobinemia has two distinct causes: biallelic cytochrome b5 reductase deficiency from CYB5R3 variants, and autosomal dominant hemoglobin M variants. Acquired methemoglobinemia is far more common and follows an oxidant exposure, with nitrites and nitrates, benzocaine and other local anesthetics, dapsone, phenazopyridine, sulfonamides, aniline dyes, chlorates, rasburicase, and inhaled nitric oxide as common causes. Glucose-6-phosphate dehydrogenase (G6PD) deficiency does not cause congenital methemoglobinemia. G6PD status is still tested when methemoglobinemia is treated, because methylene blue is contraindicated in deficiency.^{1515 Iolascon A, et al. Recommendations for diagnosis and management of… 2021.}

A methemoglobin percentage is incomplete without the total hemoglobin, because cyanosis tracks the absolute concentration of methemoglobin and appears above about 1.5 g/dL regardless of the fraction. The same percentage therefore means something different in a patient with anemia. Two tests separate inherited from acquired disease. Cytochrome b5 reductase activity distinguishes enzyme deficiency from oxidant exposure, and hemoglobin electrophoresis or targeted globin sequencing identifies a hemoglobin M variant.^{1515 Iolascon A, et al. Recommendations for diagnosis and management of… 2021.}

A saturation gap can prompt co-oximetry, especially when methemoglobinemia is suspected. Arterial pO₂ measures dissolved oxygen and can remain normal despite impaired hemoglobin oxygen carriage. A saturation calculated from pO₂ assumes ordinary hemoglobin behavior. Conventional pulse oximetry tends toward about 85% in marked methemoglobinemia, so a difference of roughly five percentage points from calculated saturation is a useful finding. With carbon monoxide exposure, however, pulse oximetry and calculated saturation can both appear normal. Neither their agreement nor a normal pO₂ excludes CO exposure. Obtain blood co-oximetry when CO exposure is suspected.^{1111 Centers for Disease Control and Prevention. Clinical guidance for carbon monoxide poisoning. 2026.} When comparing saturation results, identify whether the instrument reports functional saturation (oxyhemoglobin divided by oxyhemoglobin plus deoxyhemoglobin) or fractional saturation (oxyhemoglobin divided by all measured hemoglobin species).^{1414 Agency for Toxic Substances and Disease… Nitrate and nitrite toxicity.} Methylene blue given as treatment also interferes with co-oximetry, so a specimen collected before treatment is more useful than one collected after. At methylene blue concentrations of 50 to 70 mg/L, one widely used blood gas analyzer returned incalculable co-oximetry results.^{1616 Stevic I, et al. Determining susceptibility of routine clinical biochemistry… 2025.}

Sulfhemoglobin has sulfur incorporated into the heme, cannot be reconverted, and persists for the life of the erythrocyte. Its absorbance sits close enough to methemoglobin that many co-oximeters report it as methemoglobin, which matters because it does not respond to methylene blue. Adding cyanide to the specimen distinguishes them: cyanide converts methemoglobin to bright red cyanmethemoglobin and abolishes its 630 nm peak. Sulfhemoglobin is unchanged. A drop of blood that stays chocolate brown on exposure to air indicates a dyshemoglobin. It does not identify or quantify it.^{1414 Agency for Toxic Substances and Disease… Nitrate and nitrite toxicity.}

Cyanide inhibits mitochondrial cytochrome c oxidase, complex IV, halting oxidative phosphorylation and producing histotoxic hypoxia despite adequate oxygen delivery. The supporting laboratory pattern is a high-anion-gap lactic acidosis with a narrowed arteriovenous oxygen difference and a raised venous oxygen saturation, and no one of those findings is specific.^{1717 Agency for Toxic Substances and Disease… Toxicological profile for cyanide.} Lactate is the fastest useful measurement. In acute cyanide poisoning without fire exposure, a plasma lactate at or above 8 mmol/L was 94% sensitive and 70% specific for a blood cyanide at or above 1.0 mg/L, with specificity of 85% among patients not receiving catecholamines.^{1818 Baud FJ, et al. Value of lactic acidosis in the assessment of the severity of… 2002.} In victims of smoke inhalation without severe burns, a plasma lactate above 10 mmol/L was a sensitive indicator of a blood cyanide concentration above about 1.0 mg/L.^{1919 Baud FJ, et al. Elevated blood cyanide concentrations in victims of smoke… 1991.} Shock, burns, seizures, carbon monoxide, and catecholamine infusions all lower that specificity, so the lactate concentration supports interpretation and does not replace clinical judgment.^{1717 Agency for Toxic Substances and Disease… Toxicological profile for cyanide.}

The specimen determines how a cyanide result is interpreted. Cyanide distributes between plasma and erythrocytes and attaches more readily to plasma albumin than to hemoglobin, plasma cyanide usually runs one-third to one-half of the whole-blood concentration, and cyanide leaves plasma and serum rapidly after collection, especially in the first 20 minutes. Whole blood is the usual clinical matrix, and it is reported stable refrigerated for weeks, but stability and handling are method- and laboratory-specific: at least one reference laboratory requires fluoride-oxalate tubes with prompt freezing, so collect and handle the specimen the way the performing laboratory specifies. Whole blood, erythrocyte, and plasma results are not numerically interchangeable, so the report names the matrix and the method. Reference-laboratory turnaround can run more than a week, so a cyanide concentration confirms exposure after the fact, and early care proceeds without it.^{1717 Agency for Toxic Substances and Disease… Toxicological profile for cyanide.}

Enzymatic conversion to thiocyanate, mainly by rhodanese, is the main clearance route, and up to 80% of an administered dose leaves as urinary thiocyanate. Thiocyanate is therefore the marker used for chronic low-level exposure, but several other sources also raise it. Reported means run about 7.1 µg/mL in plasma and 12.3 µg/mL in urine for smokers against 2.0 and 2.1 µg/mL for nonsmokers, dietary thiocyanate from glucosinolates in cruciferous vegetables contributes, and sodium nitroprusside releases cyanide that is converted to thiocyanate, so prolonged or high-dose infusion prompts thiocyanate monitoring of its own.^{2020 National Library of Medicine. Sodium nitroprusside injection. 2026.} Exclude all three sources before interpreting a thiocyanate result as environmental cyanide exposure.^{1717 Agency for Toxic Substances and Disease… Toxicological profile for cyanide.}

Caustic acid and alkali ingestions have no confirmatory analyte. The laboratory contribution is blood gas, electrolyte, and anion gap monitoring, and a toxicology screen neither detects nor excludes them.

Inductively coupled plasma mass spectrometry is the routine method for these four elements, with graphite furnace atomic absorption still in service for single analytes. The trace elements lesson covers the instrumentation families and the contamination-free collection they require.

Arsenic exists in forms whose toxicity differs widely, so speciation determines the interpretation. Arsine gas is the acutely dangerous inhaled form and produces massive intravascular hemolysis with hemoglobinuria and subsequent renal injury. Inorganic trivalent and pentavalent arsenic in contaminated water or soil are the toxic exposures usually being investigated. Arsenobetaine and arsenocholine from seafood are essentially nontoxic, and they can raise a total urine arsenic several-fold after a seafood meal without any toxic exposure.^{2121 Agency for Toxic Substances and Disease… Toxicological profile for arsenic.}

Urine is the specimen, as a 24-hour collection or a creatinine-normalized random specimen. Roughly 45 to 85% of an absorbed inorganic dose appears in urine within one to three days, and whole-body clearance has a half-time near 40 to 60 hours, so the collection window is short and blood arsenic is of limited value. An elevated total urine arsenic must not be reported as exposure. Laboratories reflex to fractionation, which separates inorganic and methylated arsenic from the seafood forms, and collection follows abstinence from seafood, commonly 48 to 72 hours. Chronic exposure findings require clinical assessment. The hematologic effects are described in the anemia lesson.^{21,1321 Agency for Toxic Substances and Disease… Toxicological profile for arsenic.13 ARUP Laboratories. Carboxyhemoglobin quantitation, whole blood by co-oximetry…}

Cadmium has no physiologic role. Tobacco smoke, battery and pigment manufacturing, and food from contaminated soil are the recurring sources. Absorbed cadmium binds metallothionein, a small metal-binding protein, and the cadmium-metallothionein complex is filtered and reabsorbed by the proximal tubule, which is how the kidney becomes the organ of accumulation and injury. Retention is measured in decades, with a whole-body half-time that can exceed 26 years, so blood cadmium reflects recent exposure over months, and urine cadmium, normalized per gram of creatinine, better reflects accumulated body burden. Tubular injury is monitored by measuring low-molecular-weight proteins in urine: beta-2-microglobulin, retinol-binding protein, and N-acetyl-beta-D-glucosaminidase, alongside glucosuria and aminoaciduria. Severe chronic exposure produced the osteomalacia and osteoporosis named itai-itai disease.^{2222 Agency for Toxic Substances and Disease… Toxicological profile for cadmium.}

The commonly quoted cadmium values of 5 µg/L in blood and 5 µg/g creatinine in urine are the American Conference of Governmental Industrial Hygienists (ACGIH) biological exposure indices. They are unsuitable as reference intervals, and both carry the notation that the determinant appears at a measurable background concentration in people with no occupational exposure. In general-population surveys, blood cadmium in nonsmokers centers well below 1 µg/L, though the upper tail of the distribution reaches and exceeds 1 µg/L.^{2222 Agency for Toxic Substances and Disease… Toxicological profile for cadmium.}

Lead is carried almost entirely in erythrocytes, so testing uses whole blood collected with contamination-controlled materials. Use the receiving laboratory's approved anticoagulant and tube. EDTA is common, and some assays also accept sodium heparin. Serum is not interchangeable with whole blood.^{2727 ARUP Laboratories. Lead, Whole Blood (Venous).} The Centers for Disease Control and Prevention blood lead reference value for children is 3.5 µg/dL. It identifies children with exposure above most of their peers. It is not a toxicity threshold, and no safe blood lead concentration has been established. An elevated capillary screening result requires venous confirmation according to the applicable guidance.^{2323 Centers for Disease Control and Prevention. Recommended actions based on blood lead level; CDC updates blood…}

Mercury requires a specimen matched to each of its three forms. Swallowed elemental mercury is poorly absorbed through the gastrointestinal tract, an estimate near 0.04% for dental amalgam particles, but elemental mercury vapor is roughly 70 to 80% absorbed on inhalation, which makes inhalation the hazardous route and urine the specimen for elemental and inorganic exposure. Inorganic salts are locally corrosive and injure the kidney, and urine is again the specimen. Methylmercury from predatory fish is close to completely absorbed, crosses the blood-brain barrier as a cysteine complex, and concentrates in the brain and liver. It is eliminated mainly in feces with a blood half-time of roughly one to two months, so urine is unsuitable for assessing it and whole blood is the specimen.^{2424 Agency for Toxic Substances and Disease… Toxicological profile for mercury.}

Mercury binds protein sulfhydryl groups, which is the shared mechanism across the forms. Blood and urine methods, whether inductively coupled plasma mass spectrometry or cold-vapor atomic absorption, report total mercury and do not distinguish chemical form, so the exposure history and the specimen type indicate which form is present. Segmental hair analysis can reconstruct a methylmercury exposure timeline because hair incorporates mercury at keratinization in proportion to blood at that time. This is the one accepted use of hair testing, and it remains vulnerable to external contamination. Dimethylmercury is chiefly a laboratory safety hazard: it is rapidly absorbed through skin, and a lethal exposure occurred through a latex glove.^{2424 Agency for Toxic Substances and Disease… Toxicological profile for mercury.}

Representative adult values, all laboratory-specific and superseded by the performing laboratory's own intervals and report comments, include blood arsenic at or below 12.0 µg/L, random urine arsenic at or below 34.9 µg/L, blood cadmium at or below 5.0 µg/L, and blood total mercury at or below 10.0 µg/L.^{1313 ARUP Laboratories. Carboxyhemoglobin quantitation, whole blood by co-oximetry…}

Organophosphates and carbamates inhibit acetylcholinesterase, so acetylcholine accumulates at central, peripheral, and neuromuscular synapses. Direct measurement of the parent compound is rarely useful, because these agents are metabolized quickly and the specimen usually arrives after they are gone. Enzyme activity is used as the exposure marker, and two different enzymes can be measured.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Erythrocyte acetylcholinesterase, also called true cholinesterase, is the same enzyme found at the synapse and more closely reflects neural inhibition. Plasma or serum butyrylcholinesterase, called pseudocholinesterase, is hepatically synthesized, more widely available, falls sooner, and is the less specific exposure indicator. Their recovery rates differ, which matters for serial testing. Butyrylcholinesterase recovers as the liver resynthesizes it. One pesticide profile estimates about 25% of activity restored in the first 7 to 10 days, with activity depressed for up to about a month after severe poisoning. Erythrocyte acetylcholinesterase recovers only with erythrocyte turnover, conventionally estimated at about 1% of baseline activity per day, so it normalizes over months. Both figures are approximations that vary with the agent and the exposure.^{2525 Agency for Toxic Substances and Disease… Toxicological profile for parathion. 2017.}

Variation between individuals is wide for both enzymes, which is the main interpretive problem. A result inside the population reference interval can still represent a large fall from that person's own baseline. Occupational monitoring programs for pesticide handlers therefore require a pre-exposure baseline and act on percentage depression from that baseline. Keep serial results on one method in one laboratory, because activity assays are not interchangeable between methods.

Carbamates have a preanalytic problem that organophosphates do not. Carbamylation of the enzyme is reversible and decarbamylation proceeds spontaneously, so a carbamate-inhibited specimen can reactivate on standing, on warming, or on dilution and return a normal activity despite a real exposure. Keep the specimen cold and analyze it promptly. The erythrocyte recovery model does not apply to carbamates. Organophosphate inhibition proceeds through phosphorylation and aging and does not reverse in the tube.

Low butyrylcholinesterase has many non-pesticide causes, so exclude them before attributing a low value to exposure: hepatitis, cirrhosis and other liver disease, malnutrition, pregnancy and estrogen-containing contraceptives, acute infection, malignancy, and inherited low-activity variants. Those inherited variants are the reason for the dibucaine number, which reports the percentage by which dibucaine inhibits the patient's enzyme, a property separate from the activity itself. The usual homozygote gives a dibucaine number near 80, a heterozygote near 60, and an atypical homozygote near 20, and the test identifies the variants that prolong apnea after succinylcholine.

Urinary metabolites document exposure: dialkyl phosphates for organophosphates generally, and 3,5,6-trichloro-2-pyridinol for chlorpyrifos. They establish that absorption occurred and do not show the degree of enzyme inhibition. Paraquat has its own qualitative urine test, in which sodium dithionite in alkaline conditions produces a blue color in the presence of paraquat. The result indicates whether paraquat is present without measuring its concentration.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

These tests appear in older teaching material and are still ordered. Each one fails for a specific analytical reason.

Before you release any toxicology result, check that the specimen, the method, and the panel could have detected the substance in question.