Comprehensive Metabolic Panel (CMP)

Serum glucose reflects the balance between hepatic glucose production — glycogenolysis and gluconeogenesis, primarily regulated by glucagon — and peripheral glucose uptake, predominantly in skeletal muscle and adipose tissue, mediated by insulin-stimulated GLUT4 translocation. In the fasting state (9–12 hours after the last meal), serum glucose reflects hepatic fasting glucose output rather than postprandial absorption, providing the most stable and reproducible metabolic glucose measurement. Glucose homeostasis is maintained within a narrow range (70–99 mg/dL fasting) through the coordinated actions of insulin, glucagon, cortisol, growth hormone, and catecholamines. The hexokinase enzymatic method used in modern glucose analysers is highly specific and accurate across the clinical measurement range.

Fasting glucose is the primary first-line screen for diabetes and prediabetes alongside HbA1c in clinical practice — a fasting glucose of 100–125 mg/dL on two occasions confirms impaired fasting glucose (IFG, a prediabetes state), and ≥126 mg/dL on two occasions confirms type 2 diabetes per ADA and WHO diagnostic criteria. In the CMP, fasting glucose also reflects glucocorticoid status (cortisol excess elevates fasting glucose), renal gluconeogenic capacity, and the metabolic risk context for interpreting other panel parameters — particularly eGFR, albumin, and liver enzymes in the context of metabolic syndrome.

Urea is the principal nitrogenous waste product of hepatic amino acid catabolism — synthesised in the urea cycle from ammonium (derived from amino acid deamination) and bicarbonate in hepatocytes, and freely filtered by the renal glomerulus, with approximately 40–70% reabsorbed passively in the proximal tubule and collecting duct proportional to tubular water flow. BUN reflects the balance between hepatic urea production (driven by dietary protein intake, tissue catabolism, and gastrointestinal haemorrhage) and renal urea clearance. BUN alone is non-specific for renal dysfunction — it is confounded by protein intake, catabolic state, and hydration — making the BUN:creatinine ratio the more informative derived value for distinguishing pre-renal, intrinsic renal, and post-renal causes of azotaemia.

In the CMP, BUN is primarily assessed as one component of the BUN:creatinine ratio and in conjunction with creatinine and eGFR for renal function characterisation. Elevated BUN with normal creatinine (BUN:creatinine ratio >20:1) suggests pre-renal azotaemia — dehydration, heart failure, gastrointestinal haemorrhage, or high dietary protein intake — rather than intrinsic renal disease. BUN below 7 mg/dL may indicate severe liver failure with impaired urea cycle function, or low protein intake. In the nutritional assessment context, low BUN (combined with low albumin) suggests protein-calorie malnutrition.

Creatinine is a non-enzymatic cyclisation product of phosphocreatine catabolism in skeletal muscle, produced at a rate proportional to muscle mass and excreted almost entirely by glomerular filtration with minimal tubular secretion at physiological concentrations. It is the most widely used endogenous glomerular filtration marker in clinical practice. Serum creatinine rises as eGFR falls — but only becomes abnormally elevated when GFR has declined by 50% or more, making creatinine an insensitive early CKD marker and highlighting the superiority of eGFR (which incorporates age, sex, and race/ethnicity) for early impairment detection.

Serum creatinine within the CMP provides the raw substrate for eGFR calculation and a cross-check on eGFR reliability. Elevated creatinine with reduced eGFR confirms impaired glomerular filtration; it also screens for acute kidney injury (AKI) in hospitalised patients when compared against prior baseline. Isolated creatinine elevation without eGFR reduction may reflect increased muscle mass (athletes, bodybuilders) or high dietary meat intake rather than renal impairment. Creatinine is also elevated by certain medications including trimethoprim and cimetidine (which inhibit tubular creatinine secretion) without affecting true GFR — a clinically important drug-laboratory interaction to recognise.

eGFR is a calculated estimate of glomerular filtration rate derived from serum creatinine, age, and sex using the CKD-EPI 2021 creatinine equation — the current gold-standard clinical equation endorsed by KDIGO following the 2021 update that eliminated the race variable from the original 2009 equation. True GFR measurement requires exogenous filtration markers (inulin, iothalamate), which are impractical in routine clinical care. eGFR provides a continuous measure of glomerular filtration capacity and forms the basis of CKD staging in conjunction with albuminuria (ACR). eGFR declines physiologically with age — approximately 1 mL/min/1.73m² per year after age 40 — necessitating age-contextual interpretation.

eGFR is the cornerstone of CKD diagnosis, staging, monitoring, and pharmacological dose adjustment. Within the CMP, eGFR provides the renal function result that contextualises all other panel parameters — glucose elevation in the context of reduced eGFR suggests diabetic nephropathy; hyperkalaemia with reduced eGFR indicates impaired renal potassium excretion; hyponatraemia with reduced eGFR directs a different investigation pathway than in normal renal function. eGFR below 60 mL/min/1.73m² on two occasions at least 90 days apart confirms CKD and triggers management per KDIGO guidelines including nephrology referral consideration, medication dose adjustment, and cardiovascular risk intensification.

Sodium is the dominant extracellular cation and the primary determinant of extracellular fluid (ECF) osmolality and volume. Serum sodium is regulated by the integration of arginine vasopressin (AVP/ADH) — controlling renal free water reabsorption in the collecting duct via V2 receptors and aquaporin-2 channels — and aldosterone — driving distal tubular sodium reabsorption — in response to osmoreceptors in the hypothalamus and volume sensors in the great vessels. The kidneys can adjust urine osmolality from 50–1200 mOsm/kg to precisely maintain serum sodium within the normal range across extreme variations in water and sodium intake.

Serum sodium within the CMP provides the osmolar and volume status anchor of the metabolic assessment. Hyponatraemia (<136 mEq/L) — the most common electrolyte disorder in clinical practice — indicates free water excess relative to sodium from ADH excess (SIADH), hypothyroidism, adrenal insufficiency, heart failure, cirrhosis, or psychogenic polydipsia; severity classification and rate of correction determine management urgency. Hypernatraemia (>145 mEq/L) indicates free water deficit from diabetes insipidus, inadequate water intake, or excessive insensible losses. In outpatient functional medicine CMP interpretation, mild hyponatraemia in the 130–135 mEq/L range is an important finding requiring clinical cause investigation.

As described in the potassium standalone file (BML-MET-001) — potassium is the principal intracellular cation maintained by Na⁺/K⁺-ATPase, with 98% intracellular and only 2% extracellular. Serum potassium determines cardiac membrane potential and neuromuscular excitability. Pseudohyperkalaemia from in vitro haemolysis is the most common source of elevated potassium results in clinical practice.

In the CMP context, serum potassium is interpreted alongside sodium, bicarbonate, and creatinine/eGFR — the electrolyte-acid-base-renal pattern providing integrated physiological context. Hypokalaemia alongside metabolic alkalosis (elevated bicarbonate) suggests vomiting or diuretic use. Hyperkalaemia alongside metabolic acidosis (low bicarbonate) and elevated creatinine is consistent with CKD with impaired potassium excretion and retained acids. Hypokalaemia alongside elevated sodium and hypertension is the classic pattern of primary aldosteronism. Haemolysis must always be excluded before treating elevated potassium.

Chloride is the major extracellular anion, passively following sodium to maintain electroneutrality. It is reabsorbed in the proximal tubule, loop of Henle, and distal nephron alongside sodium. Chloride homeostasis is inextricably linked to sodium and bicarbonate balance — as bicarbonate rises (metabolic alkalosis), chloride falls to maintain electroneutrality, and vice versa.

Within the CMP, chloride is most clinically informative through its contribution to the anion gap calculation — AG = Na⁺ − (Cl⁻ + HCO₃⁻) — which identifies anion gap metabolic acidosis from lactate, ketones, uraemia, salicylates, or toxic alcohols when the AG exceeds 12 mEq/L. Hyperchloraemia with normal sodium suggests non-anion gap (hyperchloraemic) metabolic acidosis from renal tubular acidosis or gastrointestinal bicarbonate losses. Hypochloraemia alongside low sodium and high bicarbonate is consistent with metabolic alkalosis from vomiting, diuretics, or nasogastric suction.

Total CO2 in the CMP is measured by an enzymatic method on serum and primarily represents bicarbonate — the principal extracellular buffer maintaining blood pH. Bicarbonate is generated by the reaction of CO2 and water catalysed by carbonic anhydrase in erythrocytes and renal tubular cells, and is the primary renal component of acid-base regulation — the kidney excretes or retains bicarbonate to compensate for respiratory CO2 imbalances and to regulate non-volatile acid load from metabolism.

Serum total CO2 within the CMP provides the acid-base balance indicator. Low CO2 (<22 mEq/L) indicates metabolic acidosis — causes include diabetic ketoacidosis, lactic acidosis, renal tubular acidosis, diarrhoea, and advanced CKD — or respiratory alkalosis from hyperventilation. High CO2 (>29 mEq/L) indicates metabolic alkalosis — from vomiting, diuretic use, or hypokalaemia — or compensated respiratory acidosis. Combined with sodium and chloride, CO2 enables anion gap calculation. In CKD monitoring, falling bicarbonate signals progressive metabolic acidosis requiring bicarbonate supplementation per KDIGO guidelines.

Total serum calcium comprises three fractions: ionised (free) calcium — approximately 45–50%, the biologically active fraction; protein-bound calcium — approximately 40–45%, predominantly albumin-bound; and complexed calcium — approximately 10%, bound to phosphate, citrate, and bicarbonate. Total calcium measurement reflects all three fractions. The ionised fraction is tightly regulated within a narrow range (1.15–1.29 mmol/L, equivalent to 4.6–5.2 mg/dL) through the integrated actions of PTH, calcitriol (1,25-dihydroxyvitamin D), and calcitonin acting on intestinal absorption, renal reabsorption, and bone remodelling.

Serum calcium within the CMP provides the calcium homeostasis screen that triggers the PTH/vitamin D investigation pathway when abnormal. Hypercalcaemia (>10.2–10.5 mg/dL) — the most common cause of which is primary hyperparathyroidism in outpatients and malignancy in inpatients — warrants PTH measurement as the next step (see BML-THY-019). Hypocalcaemia (<8.5 mg/dL, or corrected for albumin) indicates PTH insufficiency, vitamin D deficiency, hypomagnesaemia, or hungry bone syndrome post-parathyroidectomy. In all CMP calcium interpretation, albumin must be checked simultaneously — hypoalbuminaemia lowers total calcium without affecting ionised calcium, producing false apparent hypocalcaemia that is corrected by the albumin adjustment formula.

Total serum protein is the combined mass of all circulating proteins — predominantly albumin (approximately 60%) and globulins (approximately 40% — alpha-1, alpha-2, beta, and gamma fractions). It is measured by the biuret colorimetric reaction. Total protein reflects the sum of hepatic synthetic output (albumin) and immunoglobulin production (gamma globulins), and is interpreted most informatively through protein electrophoresis when an abnormal fraction pattern is suspected.

Within the CMP, total protein is primarily interpreted in the context of albumin — total protein minus albumin estimates the globulin fraction (normal approximately 2.0–3.5 g/dL). Elevated total protein with elevated globulin (globulin fraction >3.5 g/dL) suggests hypergammaglobulinaemia from chronic inflammation, hepatic cirrhosis, autoimmune disease, or monoclonal gammopathy — the latter warrants serum protein electrophoresis (SPEP) when suspected. Low total protein with low albumin indicates protein-calorie malnutrition, chronic liver disease, malabsorption, or protein-losing enteropathy. The albumin:globulin (A:G) ratio below 1.0 is a non-specific flag for further investigation.

Albumin is a 66 kDa non-glycosylated protein synthesised exclusively by hepatocytes at a rate of 10–15 g/day, with a plasma half-life of approximately 20 days. It serves as the principal plasma oncotic protein (maintaining intravascular fluid balance), the primary carrier of non-polar molecules (fatty acids, bilirubin, calcium, drugs, hormones), and as the major blood pH buffer. Albumin synthesis is regulated by oncotic pressure, nutritional state, and hormonal signals — it is suppressed as a negative acute phase reactant during systemic inflammation, analogous to ferritin upregulation, as hepatic synthetic priority shifts to acute phase proteins.

Albumin is the most clinically informative single CMP analyte for nutritional status and hepatic synthetic function. Hypoalbuminaemia (<3.5 g/dL) indicates: chronic liver disease with impaired synthetic capacity; protein-calorie malnutrition or malabsorption; nephrotic syndrome with urinary protein loss; or protein-losing enteropathy. In chronic inflammatory states, albumin falls as a negative acute phase reactant — hypoalbuminaemia in a hospitalised patient does not necessarily indicate malnutrition; concurrent hs-CRP confirms the inflammatory component. Albumin is required to correctly interpret total calcium (albumin correction formula) and total protein (A:G ratio calculation). In functional medicine, albumin above 4.0 g/dL is the target for adequate hepatic synthetic function, nutritional status, and optimal drug binding.

Total bilirubin is the sum of unconjugated (indirect) and conjugated (direct) bilirubin in serum — the end products of haem catabolism from senescent erythrocytes processed by reticuloendothelial macrophages. Unconjugated bilirubin is transported albumin-bound to hepatocytes for glucuronide conjugation by UGT1A1; conjugated bilirubin is secreted into bile via MRP2 transporters. The CMP reports total bilirubin only — direct and indirect fractionation requires a separate order (see BML-GAS-043) when the total is elevated.

Total bilirubin within the CMP provides the first-line hepatobiliary and haemolytic screen. Elevation above 1.2 mg/dL triggers clinical assessment for jaundice — at approximately 2.5–3.0 mg/dL, scleral icterus becomes clinically apparent. In the pattern context of the full CMP: elevated total bilirubin with elevated ALT and AST suggests hepatocellular injury; elevated total bilirubin with elevated ALP (if reported) suggests cholestatic disease; elevated total bilirubin with normal liver enzymes in a young adult with mild elevation (1.5–3.0 mg/dL) is consistent with benign Gilbert's syndrome (unconjugated hyperbilirubinaemia from UGT1A1 variant — confirmed by normal direct bilirubin on fractionation). Isolated mild total bilirubin elevation in an asymptomatic patient with otherwise normal CMP is most commonly Gilbert's syndrome and requires no further investigation beyond direct bilirubin fractionation for confirmation.

ALT is a cytoplasmic enzyme present predominantly in hepatocytes — the most liver-specific aminotransferase — catalysing the reversible transfer of an amino group from alanine to alpha-ketoglutarate, producing pyruvate and glutamate. It is released into the circulation when hepatocyte cell membrane integrity is compromised by injury, inflammation, or necrosis. ALT is approximately 3–5 times more specific for hepatocellular injury than AST, which is also abundant in cardiac muscle, skeletal muscle, and kidney.

ALT is the primary hepatocellular injury marker in the CMP — the most sensitive indicator of active hepatocyte damage from any cause: viral hepatitis (acute or chronic), non-alcoholic fatty liver disease (NAFLD), alcoholic hepatitis, autoimmune hepatitis, drug-induced liver injury (DILI), ischaemic hepatitis, and metabolic causes. The magnitude of ALT elevation provides prognostic information: mild elevation (1–3× ULN) is non-specific and common in NAFLD and medication effect; moderate elevation (3–10× ULN) warrants hepatic investigation; severe elevation (>10–20× ULN) suggests acute hepatocellular necrosis from viral hepatitis, ischaemia, or toxin. The ALT:AST ratio provides aetiological clues — ratio >2:1 favours NAFLD or viral hepatitis; ratio >3:1 with high absolute values suggests alcoholic liver disease.

AST is present in hepatocytes, cardiac myocytes, skeletal muscle, kidney, and brain — making it a less liver-specific marker than ALT. In hepatocytes, AST is found in both cytoplasmic (soluble) and mitochondrial isoforms. Severe hepatocellular injury releases both isoforms; milder injury releases predominantly the cytoplasmic form. The mitochondrial AST fraction is particularly elevated in alcoholic liver disease and ischaemic hepatitis.

Within the CMP, AST is interpreted alongside ALT to characterise the hepatocellular injury pattern. An AST:ALT ratio above 2:1 — particularly with absolute AST elevation — is the classic pattern of alcoholic liver disease (Discriminant Function calculation uses AST and bilirubin). An isolated AST elevation with normal ALT suggests non-hepatic sources — cardiac injury (acute MI, myocarditis), skeletal muscle disease (rhabdomyolysis, myopathy), or thyroid disease. In NAFLD, both ALT and AST are elevated but ALT predominates (ALT:AST >1). In acute liver failure, both ALT and AST may be extremely elevated (>1000 U/L) with the AST:ALT ratio becoming less discriminating.