Chloride, Bicarbonate, Calcium, Magnesium, and Phosphate

Chloride is the main anion paired with sodium. It follows sodium through the nephron and exchanges with bicarbonate across membranes to maintain charge balance, so chloride is linked to osmolality, extracellular volume, and acid-base status. In the late proximal tubule, sodium chloride reabsorption proceeds only as fast as chloride follows, and the band 3 exchanger in the erythrocyte membrane swaps intracellular bicarbonate for plasma chloride as red cells load CO₂ in tissue, reversing the exchange in the lung. Serum chloride usually tracks sodium, but it also moves opposite bicarbonate in ways sodium does not, so review the chloride result separately from sodium.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Hyperchloremia accompanies bicarbonate loss in diarrhea and renal tubular acidosis, appears across the other normal-anion-gap acidoses, and follows chloride-rich intravenous fluid administration and pure water loss. The anion gap lesson uses that reciprocal movement to classify acid-base disorders. Hypochloremia follows gastric loss of hydrochloric acid in vomiting or drainage, renal salt wasting from diuretics or tubular disease, dilution, and high-bicarbonate states such as chronic respiratory acidosis and metabolic alkalosis.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Specimen and method. Use serum or lithium-heparin plasma. Quantitative urine chloride excretion requires a 24-hour collection, commonly refrigerated without additive. The effect of hemolysis on chloride depends on the analyzer, the method, and the protocol. In a serial-dilution study of hemolyzed serum from 12 subjects, chloride fell measurably even below a serum hemoglobin of 0.6 g/L, a degree of hemolysis that visual inspection barely detects.^{22 Lippi G, et al. Influence of hemolysis on routine clinical chemistry testing. 2006.} For example, the Atellica CH A-LYTE instructions provide hemoglobin interference data for chloride and a separate hemolyzed-specimen exclusion for potassium. Accept or reject each specimen by the relevant assay instructions and your laboratory's validated policy. A rejection rule for one assay applies to that assay alone, and a manufacturer's study bias is never applied as a correction to a patient result.^{33 Siemens Healthineers. A-LYTE Integrated Multisensor (IMT Na K Cl). 2024.} The routine method is potentiometry with a chloride-selective electrode, a solid-state silver-silver chloride or polymeric ion-exchange membrane read against a reference electrode through the Nernst relationship, as described in Tietz.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.} Coulometric titration remains in use as the specialized sweat-chloride technique standardized in CLSI C34,^{44 Clinical and Laboratory Standards Institute. Sweat Testing. 2024.} and mercurimetric titration is historical. Bromide, iodide, thiocyanate, and related anions generate false chloride readings at some electrodes and shrink the calculated anion gap, so check for them whenever the electrolytes do not fit the clinical picture.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

These are typical adult intervals at an average diet. Each laboratory verifies its own, and the clinical reference intervals lesson compares intervals across disciplines.

Bicarbonate is the second most abundant extracellular anion after chloride and the measurable core of the principal extracellular buffer system. The chemistry panel reports it as total CO₂, a measurement that captures bicarbonate, dissolved CO₂, carbonic acid, and small carbonate and protein-bound fractions. At physiologic pH roughly 95% of the total is bicarbonate, so the panel value is used as the bicarbonate result. The two are still different measurands. Total CO₂ is measured directly in venous serum or plasma. Blood-gas bicarbonate is calculated from measured pH and pCO₂, and the acid-base lesson covers this distinction.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.} Proximal tubules reclaim 80 to 90% of filtered bicarbonate by secreting hydrogen ion in a carbonic-anhydrase-dependent process. Alkalosis promotes urinary bicarbonate excretion, and acidosis raises reclamation as far as filtered load, chloride availability, and distal transport allow.

Total CO₂ is among the least stable routine analytes, because dissolved CO₂ escapes continuously from an uncapped tube. Older studies reported losses approaching 6 mmol/L per hour,^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.} and the U.S. Roche CO₂-L bicarbonate method sheet for cobas c 303/503/703 (version 4.0, September 2025) states that bicarbonate in uncapped tubes decreases by approximately 4 mmol/L after one hour. The size of the loss depends on the assay, air exposure, headspace, temperature, and elapsed time. For serum or lithium-heparin plasma separated from the cells and stored tightly stoppered, the same sheet claims stability of 40 hours at 15 to 25 °C and 7 days at 4 to 8 °C, and it makes each laboratory responsible for establishing stability criteria under its own conditions. Follow the current instructions for the assay and jurisdiction in use. Keep specimens stoppered, separate them promptly, and analyze them without avoidable delay. For interference, that sheet reports no significant effect from hemolysis, icterus, or lipemia up to its stated index limits, or from immunoglobulin up to 35 g/L, and it cautions that an IgM gammopathy such as Waldenström macroglobulinemia may in rare cases give unreliable results. Other methods set their own limits.^{55 Roche Diagnostics. CO2-L: Bicarbonate Liquid. 2025.}

Methods. Modern chemistry analyzers measure total CO₂ enzymatically. Phosphoenolpyruvate carboxylase, a magnesium-dependent enzyme, consumes specimen bicarbonate:

Malate dehydrogenase then couples the oxaloacetate to a reduced nucleotide cofactor:

The fall in reduced-cofactor absorbance is proportional to specimen bicarbonate. The readout wavelength is reagent-specific: reagents using native NADH are read near 340 nm, and other current reagents use reduced-cofactor analogs read near 405 to 415 nm. Reagent blanks absorb atmospheric CO₂, so handle reagents exactly as the insert directs. Acidification procedures that release CO₂ into a pCO₂ electrode remain only on older and specialized instruments.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

A common adult venous serum or plasma interval is 22 to 29 mmol/L. Textbook teaching sets vary around it, and each laboratory reports against its own verified interval.

Examination reference ranges. The ASCP BOC certification examination uses a separate composite set: sodium 136 to 145 mmol/L, total CO₂ 22 to 33 mmol/L, arterial pCO₂ 35 to 44 mm Hg, arterial pO₂ above 80 mm Hg, and arterial bicarbonate 23 to 29 mmol/L. These values are for exam preparation. Report patient results against your laboratory's verified interval. The complete composite set is in the examination reference ranges lesson.^{66 American Society for Clinical Pathology… MLS(ASCP) and MLS(ASCP)i Content Guideline. 2026.}

A total calcium result reports the whole circulating pool, and only part of that pool is active. About 99% of body calcium, roughly 1 kg, sits in bone. Blood calcium runs about 45 to 50% ionized Ca²⁺, the physiologically active fraction; 40 to 45% protein-bound, mostly to albumin; and the remainder complexed with bicarbonate, citrate, lactate, or phosphate, and pH shifts the distribution among the three. The ionized fraction averages about 1.18 mmol/L. Hypocalcemia depresses myocardial contractility, acute falls prolong the QT interval, and severe or rapid declines progress to tetany.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Total calcium can misrepresent calcium status whenever binding conditions change. Albumin loss lowers the protein-bound pool and the total, and ionized calcium stays unchanged. Citrate from transfused blood products chelates calcium directly. Alkalosis shifts calcium onto albumin, and phosphate excess complexes it. Ionized calcium is the measurement of choice in surgery, critical illness, and any setting with changing citrate, bicarbonate, pH, or protein. In acute illness, total calcium cannot reliably predict the ionized fraction. In stable outpatients with normal pH, renal function, and binding proteins, the widely used albumin-adjustment coefficient derived from the Payne-era work remains in use:^{77 Payne RB, et al. Interpretation of serum calcium in patients with abnormal serum… 1973.}

Corrected calcium. Suppose total calcium is 7.9 mg/dL with albumin 2.5 g/dL.

Corrected calcium = 7.9 + 0.8 × (4.0 − 2.5) = 7.9 + 1.2 = 9.1 mg/dL

The correction lifts the value into the reference interval by estimating the calcium that the missing albumin no longer binds. The 0.8 factor is the common modified albumin-adjustment form. The 1973 paper it derives from used a different coefficient. The adjustment approach dates from before ionized calcium was routinely available, although ionized-calcium measurement itself already existed. In a comparison of 5,055 samples against measured ionized calcium, corrected calcium agreed no better than unadjusted total calcium, and it often agreed worse when albumin, kidney function, or pH was abnormal.^{88 Pekar J-D, et al. Calcium state estimation by total calcium. 2020.} A corrected value near normal therefore never confirms a normal ionized fraction. When calcium status must be known accurately, measure ionized calcium.

Regulation. Parathyroid hormone, secreted by the parathyroid glands when their calcium-sensing receptors detect falling ionized calcium, raises calcium through osteoclastic bone resorption, distal renal tubular Ca²⁺ reabsorption, renal phosphate excretion, and renal 1α-hydroxylase activation. Calcitriol, 1,25-dihydroxyvitamin D, is produced when the liver and then the kidney hydroxylate dietary vitamin D₂ or D₃ or cutaneous D₃. It raises intestinal calcium and phosphate absorption and cooperates with PTH on bone. Calcitonin from thyroid C cells lowers calcium by inhibiting osteoclasts and promoting renal calcium and phosphate excretion. Its basal role in healthy adults is minor. The calcium-sensing receptor sets the parathyroid set point, and FGF23 from osteocytes also regulates phosphate. The parathyroid and vitamin D lesson develops both.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Hypocalcemia and hypercalcemia

Perioperative and ICU monitoring favors ionized calcium because citrated blood products, bicarbonate infusions, and acute protein shifts make calculated corrections unreliable. In healthy term neonates, ionized calcium peaks at birth, falls to a nadir in the first one to two days, and reaches its postnatal range in one to two weeks as parathyroid and vitamin D function mature.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Determination. Total calcium is measured in serum or lithium-heparin plasma collected with minimal tourniquet time, because prolonged venous stasis hemoconcentrates the protein-bound fraction.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.} EDTA, citrate, and oxalate chelate calcium, so reject specimens collected in them. Citrate contamination from blood-bank tubes or infusion lines lowers measured calcium the same way. Ionized calcium requires anaerobic handling: CO₂ loss raises pH, shifts calcium onto albumin, and falsely lowers the ionized result. Balanced calcium-titrated heparinized whole blood is the preferred specimen. Some laboratories also validate serum or plasma for ionized calcium, so check which specimens your laboratory's procedure accepts. Unbalanced liquid heparin binds calcium. IFCC guidance limits ordinary sodium or lithium heparin to 15 IU/mL of blood, permits calcium-titrated heparin below 50 IU/mL, and leaves the residual bias dependent on formulation, syringe fill, and pH.^{99 Boink ABTJ, et al. IFCC recommendation on sampling, transport, and storage for the… 1991.}

Total calcium is measured colorimetrically. Current dye systems include o-cresolphthalein complexone, shielded from Mg²⁺ by 8-hydroxyquinoline, and arsenazo III. Calibration traces to isotope-dilution inductively coupled plasma mass spectrometry (ICP-MS) reference measurement procedures, which replaced the atomic absorption spectrophotometry used historically. Hemolysis, icterus, and lipemia affect the colorimetric reactions within method-specific limits, which the analyzer's interference flags report. Ionized calcium is measured by direct potentiometry with a calcium-selective ion-selective electrode (ISE), usually alongside pH, following International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) and Clinical and Laboratory Standards Institute (CLSI) practice.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

These rows illustrate one teaching set. Intervals differ by age, method, specimen, and, for ionized calcium, sample pH. Each laboratory verifies and reports against its own.

A normal serum magnesium can occur with depleted body stores, because very little of the body's magnesium circulates. A 70-kg adult holds about 1 mol (24 g) of Mg²⁺, the fourth most abundant body cation and the second most abundant inside cells behind potassium. Bone holds roughly 53% and muscle with other soft tissue 46%, leaving under 1% in serum and erythrocytes. Serum Mg²⁺ divides about one-third protein-bound, mostly to albumin; 55 to 65% free and active; and the rest complexed. Mg²⁺ is a required cofactor for enzymes across glycolysis, ion pumping, neuromuscular transmission, and macromolecular synthesis. Estimates of the number of these enzymes range from more than 300 in classical sources to around 600 in current reviews.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Regulation. Intestinal absorption takes up roughly one-third to one-half of dietary intake, adjusting with need. The kidney is the principal regulator. The proximal tubule reclaims only 10 to 30% of filtered Mg²⁺, the thick ascending limb of Henle 50 to 70%, and the distal convoluted tubule the final tightly controlled 5 to 10%, so normally under 5% of the filtered load reaches urine. Normal serum levels sit near the kidney's excretion set point of roughly 0.6 to 0.85 mmol/L, so a slight excess spills promptly into urine. PTH may influence renal and intestinal handling. Magnesium has no single dominant hormonal regulator.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Hypomagnesemia and hypermagnesemia

Hypomagnesemia is usually silent above about 0.5 mmol/L, a commonly used severe-depletion threshold. Below it, manifestations can be cardiovascular (arrhythmia, hypertension, potentiated digitalis toxicity), neuromuscular (tremor, tetany, seizure), psychiatric (agitation, psychosis), or metabolic. Hypomagnesemia accompanies roughly 40% of hypokalemic hospital patients, and renal potassium wasting continues until magnesium is repleted, which makes hypokalemia refractory in the meantime. Hypocalcemia, hypophosphatemia, and hyponatremia often accompany the deficit. Hypermagnesemia brings hypotension, bradycardia, flushing, and nausea as levels approach 2 to 3 mmol/L, deep-tendon reflexes fade between roughly 3.5 and 5 mmol/L, and ECG changes, heart block, and respiratory paralysis appear above 5 mmol/L, with levels above 6 to 7 mmol/L life-threatening. Individual thresholds vary. Acute magnesium elevation also suppresses PTH through the calcium-sensing receptor and can cause hypocalcemia.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Determination. Use nonhemolyzed serum or lithium-heparin plasma, separated promptly: erythrocytes carry roughly three times the Mg²⁺ concentration of plasma, so hemolysis or delayed separation falsely raises the result. Oxalate, citrate, and EDTA chelate magnesium, so reject specimens collected in them. Routine methods bind a dye (calmagite, formazan derivatives, or methylthymol blue) with calcium masked so it does not react. Isotope-dilution ICP-MS procedures have succeeded atomic absorption spectrophotometry as the reference approach, and enzymatic assays exist but remain uncommon. Serum magnesium can be within its reference interval when total body stores are as much as 20% low, because under 1% of body Mg²⁺ circulates extracellularly. A normal serum magnesium therefore never excludes magnesium depletion.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

This representative adult colorimetric interval is method-specific, and each laboratory verifies its own.

Inorganic phosphate builds nucleic acids, coenzymes, and the high-energy phosphates ATP, creatine phosphate, and phosphoenolpyruvate. Severe deficiency lowers cellular ATP and lowers red-cell 2,3-DPG, left-shifting the oxyhemoglobin curve and impairing O₂ delivery. Roughly 85% of body phosphate sits in bone, and phosphate is the dominant intracellular anion.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Regulation. PTH lowers serum phosphate by increasing renal excretion, and FGF23 from osteocytes, the other major phosphaturic hormone, acts in parallel and also suppresses calcitriol synthesis. The parathyroid and vitamin D lesson covers that axis. Vitamin D raises phosphate through intestinal absorption. Growth hormone, acting through IGF-1, reduces renal excretion, so GH excess produces hyperphosphatemia. Insulin-driven glucose uptake during refeeding, and alkalemia-stimulated glycolysis in acute respiratory alkalosis, pull phosphate into cells as phosphorylated intermediates accumulate, and these transcellular shifts complicate acute interpretation.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Hypophosphatemia and hyperphosphatemia

Hypophosphatemia affects 1 to 5% of hospitalized patients and a far larger share of ICU patients, with published rates rising as high as 80% where cutoffs and populations differ. Recurring settings include diabetic ketoacidosis and its treatment, chronic obstructive pulmonary disease and asthma, malignancy, prolonged parenteral nutrition, inflammatory bowel disease, anorexia nervosa, alcoholism, and refeeding after starvation. Mechanisms are increased renal excretion, as in hyperparathyroidism; reduced intestinal absorption, as in vitamin D deficiency or phosphate-binding antacids; and transcellular shift into cells. Severe depletion below 0.3 mmol/L (1.0 mg/dL) is commonly treated as critical and accompanies respiratory failure, hemolysis, and other serious complications, although mortality associations are confounded by the underlying illness.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Hyperphosphatemia chiefly reflects failed renal excretion. Other contributors are excess intake or absorption, as when infants drink unmodified cow's milk, whose phosphate load exceeds what immature neonatal kidneys clear (regulated cow-milk-based formulas carry moderate loads and do not by themselves cause pathologic hyperphosphatemia); cellular release in tumor lysis, extensive muscle injury, in-vivo hemolysis, or lymphoblastic leukemia, whose blasts carry up to about four times the phosphate of mature lymphocytes; and hypoparathyroidism, which removes PTH-driven renal excretion. In-vitro hemolysis raises the measured value without any in-vivo change. The specimen requirements below address this.

Determination. Use serum or lithium-heparin plasma, promptly separated: erythrocytes hold abundant organic phosphate, so hemolysis or delayed separation falsely elevates the result. Tube requirements are method-specific. Many direct phosphomolybdate procedures validate EDTA plasma. Citrate and oxalate are commonly rejected, so collect what your laboratory's instructions specify. Serum phosphate follows a diurnal and postprandial rhythm with a late-morning nadir and a nocturnal peak, so standardize collection timing when you compare serial results. A 24-hour collection measures daily phosphorus excretion. Other renal phosphate assessments use the specimen and paired measurements specified by their validated protocol. For example, Mayo Clinic Laboratories offers tubular reabsorption testing with random urine and serum.^{1212 Mayo Clinic Laboratories. Tubular Reabsorption of Phosphorus, Random Urine and Serum.} The method complexes inorganic phosphate with ammonium molybdate and reads the phosphomolybdate complex directly by UV absorbance near 340 nm or reduces it to molybdenum blue read at 600 to 700 nm. Lipemia, icterus, and paraproteins interfere within method-specific limits, and paraproteins can bias phosphate in either direction.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

These rows illustrate one teaching set at adult values. Manufacturers publish narrower age- and sex-specific pediatric bands, and each laboratory verifies its own.

Each institution sets its own critical limits for these five analytes. Chloride and total CO₂ appear on few critical-value lists. Calcium, magnesium, and phosphate commonly carry limits near total calcium 6.0 or 13.0 mg/dL, magnesium 1.0 or 9.0 mg/dL, and phosphate 1.0 mg/dL. Work from your laboratory's medical-director-approved list, with direct person-to-person notification, read-back, and documentation.^{1010 Clinical and Laboratory Standards Institute. Management of Critical- and Significant-Risk Results. 2015.} Under CLIA, laboratories verify or establish the intervals they report against, and the quality control lesson describes the general QC and calibration program.^{1111 Centers for Medicare & Medicaid Services. 42 CFR § 493.1253.}

Sodium, chloride, and total CO₂ move in predictable relationships, so rule out an analytic explanation for a discordant combination before looking for a clinical one. A chloride result that moves with sodium and its other water-balance partners needs no special review. When chloride is discordant with sodium, the anion gap, or the clinical picture, review halide and drug interference, check the electrode, and recalibrate. The preanalytic quality lesson describes contamination and handling patterns. The table lists combinations seen often at the bench.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Review the electrolyte panel as a set of related results. When an expected relationship does not hold, check the specimen and the analyzer first.