Vitamins and Nutrition

Direct measurement quantifies the vitamin or its active form. It dominates current practice, and liquid chromatography with tandem mass spectrometry (LC-MS/MS) has displaced most of the older high-performance liquid chromatography (HPLC), microbiologic, and spectrophotometric procedures for these analytes, though HPLC remains in service for the tocopherols and immunoassay for cobalamin, folate, and routine 25-hydroxyvitamin D.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Functional measurement quantifies a vitamin-dependent enzyme activity, usually as an activation coefficient: the enzyme is assayed with and without added cofactor, and a large increase on addition means the cell was short of cofactor. Erythrocyte transketolase for thiamine and erythrocyte glutathione reductase for riboflavin both work this way. Functional tests reflect cellular status, which is their advantage and also why they are hard to standardize. The glutathione reductase coefficient cannot be interpreted in glucose-6-phosphate dehydrogenase deficiency, because the enzyme system it measures is already abnormal.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Metabolite measurement quantifies a substrate that accumulates when the dependent reaction slows. Methylmalonic acid and total homocysteine serve this role for cobalamin and folate. A metabolite reflects intracellular sufficiency only when nothing else in the patient raises the same metabolite, which limits interpretation of both markers.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Load and saturation tests, in which a dose is given and urinary excretion measured afterward, are historical research methods and are no longer offered clinically.

Vitamin D, cobalamin, and folate have their own sections below. Intervals in the table come from published national reference laboratory catalogs so that magnitude and units are concrete. Each interval applies to its stated method. Interpret patient results with the performing laboratory's verified values and report comments.^{2,32 ARUP Laboratories test directory. Vitamin B1 (thiamine), whole blood; vitamin B6 (pyridoxal… 2026.3 Mayo Clinic Laboratories test catalog. Vitamin A, serum; homocysteine, total, plasma.}

Most errors in these assays occur before analysis. Recollect a degraded specimen, because no calculation can recover an analyte that oxidized in the tube.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Light protection is analyte-specific and method-specific. On ARUP Laboratories' menu, pyridoxal 5'-phosphate, ascorbate, and erythrocyte folate require protection from light. 25-hydroxyvitamin D does not, and it tolerates 24 hours at ambient temperature.^{22 ARUP Laboratories test directory. Vitamin B1 (thiamine), whole blood; vitamin B6 (pyridoxal… 2026.} Serum retinol at another national laboratory is transferred to a light-protected vial and is then stable for 28 days refrigerated.^{33 Mayo Clinic Laboratories test catalog. Vitamin A, serum; homocysteine, total, plasma.} CDC's folate procedure states the size of the effect: folate exposed to light for more than 8 hours can degrade by 10 to 20%, which is why that laboratory processes folate specimens under yellow light and freezes them promptly when analysis is not immediate.^{77 Centers for Disease Control and Prevention, et al. Laboratory procedure manual. 2021.} Follow each assay's own procedure.

Temperature and elapsed time also affect stability. Ascorbate is stable only when frozen. For ARUP's whole-blood thiamine diphosphate assay, the specimen must be frozen within 1 hour of collection. Refrigerated and room-temperature specimens are unacceptable for that assay. Frozen stability is 6 months. Methylmalonic acid requires separation within about 2 hours and frozen transport, with room-temperature specimens unacceptable.^{22 ARUP Laboratories test directory. Vitamin B1 (thiamine), whole blood; vitamin B6 (pyridoxal… 2026.} Fasting also matters: nonfasting pyridoxal 5'-phosphate and retinol can reflect the last meal or dose more than status, and any nutrient measured after replacement has begun reflects the replacement.^{2,32 ARUP Laboratories test directory. Vitamin B1 (thiamine), whole blood; vitamin B6 (pyridoxal… 2026.3 Mayo Clinic Laboratories test catalog. Vitamin A, serum; homocysteine, total, plasma.}

25-hydroxyvitamin D is the status analyte, with a half-life of 2 to 3 weeks that reflects combined supply from skin synthesis, diet, and supplements. 1,25-dihydroxyvitamin D has a half-life near 4 hours, is kept within its reference interval by parathyroid hormone, calcium, and phosphate, and is often normal or high in a frankly deficient patient.^{88 Holick MF, et al. Evaluation, treatment, and prevention of vitamin D deficiency. 2011.} The International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) Committee on Bone Metabolism limits its use to specific indications: working up hypercalcemia, evaluating calcipenic rickets and osteomalacia, and separating those from FGF23-mediated phosphopenic rickets. It is not recommended for routine monitoring in chronic kidney disease, no reference measurement procedure exists for it, and the automated immunoassays that measure it do not resolve the D₂ and D₃ forms.^{99 Makris K, et al. Recommendations on the measurement and the clinical use of… 2021.} One catalog reports it by chemiluminescent immunoassay at 19.9 to 79.3 pg/mL and states that it is not an assessment of vitamin D deficiency.^{22 ARUP Laboratories test directory. Vitamin B1 (thiamine), whole blood; vitamin B6 (pyridoxal… 2026.} Synthesis, activation, and the parathyroid relationship are covered in the parathyroid and vitamin D lesson.

The reference measurement system. CDC's Vitamin D Reference Laboratory operates a higher-order procedure for total 25-hydroxyvitamin D that quantifies the D₂ and D₃ forms separately by liquid chromatography with tandem mass spectrometry, is unaffected by other isomers including C3-epi-25-OH-D₃, is calibrated against NIST standard reference material SRM 2972a, and has a maximum allowable bias of 1.7% with 5% imprecision.^{1010 Centers for Disease Control and Prevention. Vitamin D reference laboratory. 2024.} The published candidate procedure uses isotope dilution on a pentafluorophenylpropyl phase to obtain baseline separation of 25-OH-D₂ and 25-OH-D₃ from the C3 isomers, which is most important in infants, in whom the epimer makes up the largest share of measured D₃.^{1111 Mineva EM, et al. A candidate reference measurement procedure for quantifying… 2015.}

These programs have different purposes. The NIH Office of Dietary Supplements established the Vitamin D Standardization Program in 2010 and coordinated it through 2018. NIST and Ghent University supplied the independent reference measurement procedures recognized by the Joint Committee for Traceability in Laboratory Medicine.^{1212 National Institutes of Health Office of… Vitamin D Standardization Program (VDSP) key collaborators.} CDC runs the Vitamin D Standardization-Certification Program, which certifies routine total 25-OH-D assays with mean bias within 5% and imprecision below 10%.^{1313 Centers for Disease Control and Prevention. Improving performance.}

Immunoassay against LC-MS/MS. An automated immunoassay reports one total value from antibody binding after the analyte is released from vitamin D binding protein, and antibodies do not recognize the D₂ and D₃ forms equally. That unequal cross-reactivity is the main reason two methods disagree on one specimen, and it matters most in a patient taking ergocalciferol. A published comparison of automated immunoassays against LC-MS/MS found method-dependent bias across platforms and poor agreement with total 25-OH-D for one assay that measured only the D₃ form.^{1414 Farrell CJ, et al. State-of-the-art vitamin D assays. 2012.} Keep serial results for one patient on one method.

Binding protein and free fractions. Less than 0.1% of 25-OH-D circulates free, and the bioavailable fraction is the free plus albumin-bound portion. Both can be calculated from total 25-OH-D, albumin, and binding protein. Routine free or bioavailable measurement is not recommended, because those assays lack standardization and agreed reference data. Pregnancy, liver disease, nephrotic syndrome, and critical illness shift binding protein and therefore shift total 25-OH-D without necessarily changing the free fraction.^{99 Makris K, et al. Recommendations on the measurement and the clinical use of… 2021.}

The cutoffs are decision limits, and they are contested. The 2011 Endocrine Society guideline defined deficiency below 20 ng/mL and insufficiency at 21 to 29 ng/mL, and 20 ng/mL remains a common laboratory cutoff.^{88 Holick MF, et al. Evaluation, treatment, and prevention of vitamin D deficiency. 2011.} The 2011 Institute of Medicine dietary reference intake report used the same number for a different purpose, as the concentration meeting the requirement of nearly all people under minimal sun exposure, which is a population intake benchmark.^{1515 Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. 2011.} The 2024 Endocrine Society guideline found no trial evidence defining an optimal target and suggests against routine 25-OH-D testing in generally healthy adults who have no established indication, and it sets no sufficiency thresholds for that population.^{1616 Demay MB, et al. Vitamin D for the prevention of disease. 2024.} Converting units uses the 400.6 g/mol molar mass of 25-hydroxyvitamin D₃: 1 ng/mL is about 2.5 nmol/L.

Routine serum cobalamin and folate are competitive-binding chemiluminescent or electrochemiluminescent immunoassays. The cobalamin assay uses intrinsic factor as the binding protein and the folate assay uses folate-binding protein. Radioassay formats are no longer used.^{1,21 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.2 ARUP Laboratories test directory. Vitamin B1 (thiamine), whole blood; vitamin B6 (pyridoxal… 2026.}

The intrinsic-factor design has a specific failure mode. A patient with pernicious anemia may carry anti-intrinsic-factor antibody that interferes with the assay's own intrinsic factor reagent and gives a normal or high total cobalamin in a profoundly deficient patient. Reagent formulations are designed to suppress the effect, and it has still been reported repeatedly.^{17,1817 Yang DT, et al. Spurious elevations of vitamin B12 with pernicious anemia. 2012.18 Carmel R, et al. Failures of cobalamin assays in pernicious anemia. 2012.} A normal total cobalamin therefore does not exclude deficiency when the blood film, the clinical picture, or the metabolites disagree with it. Repeat on a different assay design, and measure methylmalonic acid.^{1919 Sobczyńska-Malefora A, et al. Vitamin B12 status in health and disease. 2021.}

Cobalamin and folate test selection. The College of American Pathologists (CAP) test-ordering guidance directs total cobalamin for anemia or new neurologic signs without an alternative explanation, alongside a complete blood count (CBC), reticulocyte count, lactate dehydrogenase (LDH), haptoglobin, bilirubin, and a blood-film review. Total folate is ordered only when the history supports a deficient diet or a malabsorptive comorbidity. When either concentration is borderline or discordant with clinical judgment, the metabolites are measured next: methylmalonic acid and homocysteine for cobalamin, homocysteine for folate, with intrinsic-factor antibody to confirm pernicious anemia. The guidance lists three tests as rarely indicated: serum folate in a patient with an adequate diet and no malabsorption, erythrocyte folate, and active cobalamin.^{2020 College of American Pathologists. Test ordering guidelines for suspected vitamin B12 and folate…}

Methylmalonic acid rises in intracellular cobalamin deficiency and stays normal in isolated folate deficiency, so it distinguishes the two deficiencies.^{2020 College of American Pathologists. Test ordering guidelines for suspected vitamin B12 and folate…} Renal impairment is the dominant non-nutritional cause of elevation, with age, hypovolemia, and small intestinal bacterial overgrowth contributing, so interpret an elevated value with renal function.^{1919 Sobczyńska-Malefora A, et al. Vitamin B12 status in health and disease. 2021.} One catalog measures it by LC-MS/MS with an interval of 0.00 to 0.40 µmol/L.^{22 ARUP Laboratories test directory. Vitamin B1 (thiamine), whole blood; vitamin B6 (pyridoxal… 2026.}

Total homocysteine rises in both deficiencies, and also in vitamin B₆ deficiency, renal failure, hypothyroidism, and common MTHFR variants, so it is the less specific of the two.^{20,1920 College of American Pathologists. Test ordering guidelines for suspected vitamin B12 and folate…19 Sobczyńska-Malefora A, et al. Vitamin B12 status in health and disease. 2021.} Its preanalytic requirement is strict and frequently missed: erythrocytes keep exporting homocysteine into plasma after collection, so place whole blood on wet ice and separate plasma promptly. One national laboratory allows up to 4 hours only if the specimen stays on wet ice continuously, and 1 hour otherwise. A delayed, unchilled specimen produces a falsely elevated result that can be mistaken for disease.^{33 Mayo Clinic Laboratories test catalog. Vitamin A, serum; homocysteine, total, plasma.}

Serum against erythrocyte folate. Serum folate indicates short-term status and erythrocyte folate long-term status, because folate enters the red cell during erythropoiesis and stays for the cell's life.^{77 Centers for Disease Control and Prevention, et al. Laboratory procedure manual. 2021.} Hemolysis falsely raises serum folate, because erythrocytes hold far more folate than plasma does, so interpret a hemolyzed specimen with caution or reject it. Never report it at face value.^{77 Centers for Disease Control and Prevention, et al. Laboratory procedure manual. 2021.} Erythrocyte folate measurement depends on that same gradient, which also makes it difficult to perform. In CDC's survey method, one part fresh EDTA whole blood is diluted with ten parts 1% ascorbic acid, a 1 in 11 hemolysate that is frozen promptly to hold folate in the reduced state, and a hematocrit is measured at the time of collection for the calculation.^{77 Centers for Disease Control and Prevention, et al. Laboratory procedure manual. 2021.} The reported erythrocyte folate is a calculated value.

Suppose whole-blood folate, already corrected for the method's hemolysate dilution, is 300 µg/L, serum folate on the same draw is 8 µg/L, and hematocrit is 0.40.

erythrocyte folate = [whole-blood folate − serum folate × (1 − Hct)] ÷ Hct

erythrocyte folate = [300 µg/L − (8 µg/L × 0.60)] ÷ 0.40 = 295.2 µg/L ÷ 0.40 = 738 µg/L

The subtraction removes the folate contributed by the non-erythrocyte fraction and the division converts the remainder to a per-red-cell concentration. The result combines three separate measurements and one method-specific dilution factor, each with its own imprecision, and the hematocrit must come from the same draw. The combined imprecision brings no matching gain in diagnostic value, so serum folate is the routine test.^{20,720 College of American Pathologists. Test ordering guidelines for suspected vitamin B12 and folate…7 Centers for Disease Control and Prevention, et al. Laboratory procedure manual. 2021.}

Folate testing changed after US grain fortification. FDA's final rule required folic acid in enriched cereal-grain products, with compliance from January 1, 1998.^{2121 US Food and Drug Administration. Food standards. 1996.} Deficiency became uncommon enough that CAP states it is not necessary to measure folate in a patient with an adequate diet and no evidence of malabsorption.^{2020 College of American Pathologists. Test ordering guidelines for suspected vitamin B12 and folate…}

Antibodies help explain the cause of cobalamin deficiency. Intrinsic-factor antibody is highly specific for pernicious anemia and insensitive, so a negative result does not exclude it. Gastric parietal-cell antibody is more sensitive and much less specific. The limited performance of both, together with the absence of a robust absorption test since radiolabeled cobalamin studies left service, is why the cause of a depleted cobalamin status is now inferred from antibodies and history.^{1919 Sobczyńska-Malefora A, et al. Vitamin B12 status in health and disease. 2021.} The National Institute for Health and Care Excellence (NICE) guideline NG239 treats an indeterminate total or active cobalamin as unresolved pending further testing, directs that the laboratory's own validated reference range applies to a methylmalonic acid result, notes that some antibody tests are unreliable after a recent cobalamin injection, and advises collecting before replacement whenever that does not delay treatment for megaloblastic anemia or neurologic signs. NG239 is United Kingdom guidance. A United States laboratory reports against its own verified interval and its own indeterminate-zone comment.^{2222 National Institute for Health and Care… Vitamin B12 deficiency in over 16s. 2024.}

Recent supplementation, transfusion, and food intake can change the interpretation of cobalamin or folate results. Record their timing. Supplementation may raise the measured concentration before a deficiency is fully corrected, transfusion can obscure the untreated baseline, and recent intake particularly affects serum folate. Obtain pretreatment specimens when clinically appropriate and interpret or repeat testing according to the assay and clinical question.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.} Nitrous oxide inactivates cobalamin without changing the blood concentration, and it raises homocysteine and methylmalonic acid.^{2020 College of American Pathologists. Test ordering guidelines for suspected vitamin B12 and folate…} Folate repletion can correct the anemia of cobalamin deficiency while demyelinating injury continues, so cobalamin status is measured together with folate.^{20,1920 College of American Pathologists. Test ordering guidelines for suspected vitamin B12 and folate…19 Sobczyńska-Malefora A, et al. Vitamin B12 status in health and disease. 2021.}

Vitamin K is the cofactor for hepatic gamma-carboxylation of factors II, VII, IX, and X and of proteins C and S. Warfarin inhibits the same vitamin K cycle, so check the anticoagulant history before reading any result as nutritional.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

The prothrombin time is an accessible but nonspecific functional index. It responds only after factor activity has fallen substantially, so it detects established depletion and misses developing deficiency, and it changes for many reasons unrelated to vitamin K.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.} PIVKA-II, protein induced by vitamin K absence or antagonism-II, is the undercarboxylated prothrombin also called des-gamma-carboxyprothrombin, and it appears earlier because it reflects the carboxylation defect directly. Its non-nutritional causes limit interpretation: warfarin, cholestasis, parenchymal liver disease, and hepatocellular carcinoma all raise it, and its dominant published use is as a hepatocellular carcinoma marker interpreted with other biomarkers.^{2323 Dong L, et al. Protein induced by vitamin K absence or antagonist II. 2023.} Direct phylloquinone measurement exists and reflects recent intake more than status.

Albumin has a half-life of about 20 days, transferrin 8 days, transthyretin (reported as prealbumin) 2 to 3 days, and retinol-binding protein roughly 12 hours. All four are negative acute-phase proteins whose concentrations fall with inflammation regardless of intake. The American Society for Parenteral and Enteral Nutrition (ASPEN) interprets these proteins as indicators of inflammation and the risk of an adverse outcome.^{2424 Evans DC, et al. The use of visceral proteins as nutrition markers. 2021.} They are unsuitable as nutrition markers or proxies for total body protein or muscle mass, are excluded from malnutrition definitions, and have no defined role in monitoring the delivery or efficacy of nutrition support.^{2525 American Society for Parenteral and Enteral… Appropriate use of visceral proteins in nutrition screening and…} In hospitalized adults receiving enteral nutrition, prealbumin change correlated with change in C-reactive protein and did not differ between patients meeting at least 60% of calorie needs and those falling short.^{2626 Davis CJ, et al. The use of prealbumin and C-reactive protein for monitoring… 2012.} The Society of Critical Care Medicine (SCCM) and ASPEN guideline states that these proteins are not validated for assessing protein adequacy in critical illness.^{2727 Taylor BE, et al. Guidelines for the provision and assessment of nutrition support… 2016.}

Each protein has distinct confounders. Albumin falls with hepatic disease, nephrotic syndrome, protein-losing enteropathy, capillary leak, and dilution, and it rises artifactually with venous stasis, measurably within a few minutes of tourniquet application.^{2828 Lippi G, et al. Influence of short-term venous stasis on clinical chemistry… 2005.} Transferrin concentration rises in iron deficiency, so a high transferrin can reflect iron depletion with no change in nutritional status. Iron studies are explained in the anemia lesson.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.} Prealbumin and transferrin are measured by quantitative immunoturbidimetry or immunonephelometry, and catalog intervals are method- and age-specific: one laboratory reports prealbumin at 20 to 40 mg/dL, transferrin at 200 to 360 mg/dL, and retinol-binding protein at 3.0 to 6.0 mg/dL.^{22 ARUP Laboratories test directory. Vitamin B1 (thiamine), whole blood; vitamin B6 (pyridoxal… 2026.}

No laboratory value diagnoses malnutrition. Under the Global Leadership Initiative on Malnutrition criteria, a patient screened at risk requires at least one phenotypic criterion, unintentional weight loss, low body mass index, or reduced muscle mass, together with at least one etiologic criterion, reduced intake or assimilation or disease burden with inflammation, and severity is graded from the phenotypic criteria. C-reactive protein appears only as a supportive measure for the inflammation criterion, never as a criterion itself.^{2929 Cederholm T, et al. GLIM criteria for the diagnosis of malnutrition. 2019.} The Academy of Nutrition and Dietetics and ASPEN consensus is built the same way, on two or more of six clinical characteristics: insufficient energy intake, weight loss, loss of muscle mass, loss of subcutaneous fat, fluid accumulation, and diminished grip strength. Serum proteins are not among them.^{3030 White JV, et al. Consensus statement. 2012.}

The laboratory's role is narrower. It identifies a specific micronutrient deficiency and provides the C-reactive protein needed to interpret a low nutrient marker.^{44 Berger MM, et al. ESPEN micronutrient guideline. 2022.} It also supplies the serial results used to detect refeeding syndrome. ASPEN defines the syndrome by a 10 to 20%, 20 to 30%, or greater than 30% fall in serum phosphate, potassium, or magnesium within 5 days of reintroducing calories and recommends baseline testing with repeat measurement every 12 hours through the first 3 days in high-risk adults. A thiamine concentration is not used to decide urgent clinical replacement. The nutrition service is responsible for feeding and replacement decisions.^{3131 da Silva JSV, et al. ASPEN consensus recommendations for refeeding syndrome. 2020.} Nitrogen balance from a timed urine urea nitrogen collection remains orderable, with one laboratory's excretion interval of 12.0 to 20.0 g/day. The calculation requires a complete collection and an accurate intake record, and an error in either invalidates it.^{22 ARUP Laboratories test directory. Vitamin B1 (thiamine), whole blood; vitamin B6 (pyridoxal… 2026.}

Among the analytes here, 25-hydroxyvitamin D has the most developed standardization system, with a certification program, a higher-order reference procedure, and NIST reference material behind it.^{10,1310 Centers for Disease Control and Prevention. Vitamin D reference laboratory. 2024.13 Centers for Disease Control and Prevention. Improving performance.} Folate has certified reference materials as well: the National Institute for Biological Standards and Control (NIBSC) supplies a WHO reference material for serum folate certified by LC-MS/MS and one for whole-blood folate whose value is consensus-assigned and mainly representative of the microbiologic assay, which is itself a caution about transferring whole-blood folate values across methods.^{77 Centers for Disease Control and Prevention, et al. Laboratory procedure manual. 2021.} For most of the remaining analytes, results are method-specific, and alpha-tocopherol, thiamine diphosphate, and total cobalamin each have their own interval. Keep serial monitoring in the same laboratory on the same method.^{22 ARUP Laboratories test directory. Vitamin B1 (thiamine), whole blood; vitamin B6 (pyridoxal… 2026.}

Under the Clinical Laboratory Improvement Amendments (CLIA), a laboratory using an unmodified FDA-cleared or approved nonwaived system verifies accuracy, precision, and reportable range, and confirms that the manufacturer's reference interval fits its own specimen type and patient demographics before reporting. A modified system, one that is not cleared, or one whose manufacturer specifies no interval requires the laboratory to establish the interval itself, and either activity is documented.^{3232 Centers for Medicare & Medicaid Services. CLIA proficiency testing regulations and calibration… 2025.} The quality control lesson covers the general calibration and control program.

A nutritional test report may include both a reference interval and a decision limit. The interval describes values in a healthy reference population tested by one method. The decision limit marks a threshold for action. The report needs to distinguish them even when they appear side by side. The 20 ng/mL vitamin D cutoff, the 20 and 10 µg/dL retinol thresholds, and a folate adequacy cutoff are decision limits. The tocopherol, thiamine diphosphate, and pyridoxal 5'-phosphate ranges above are reference intervals.^{15,1615 Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. 2011.16 Demay MB, et al. Vitamin D for the prevention of disease. 2024.}

An undirected multivitamin panel is also inappropriate. Each analyte in it has its own specimen, timing, and inflammation requirements, so results from a panel ordered without a clinical question cannot be interpreted. Start every vitamin request from the clinical question, then choose the compartment and specimen that fit it.

Vitamin D synthesis, activation, and the parathyroid relationship are covered in the parathyroid and vitamin D lesson. Megaloblastic morphology and the metabolic trap linking cobalamin and folate are in the anemia lesson. Biotin interference in immunoassay design is covered in endocrine methods. Albumin and prealbumin methods, dye-binding bias, and the electrophoretic fractions are in plasma proteins. Essential trace elements and contamination-free collection are in trace elements. Ascorbate interference with reagent-strip chemistry is in urine reagent strips. Cross-discipline interval comparison is in clinical reference intervals.