Clinical biochemistry: organ function tests

In short

Organ function tests are panels of blood, serum and urine analytes whose concentration or enzyme activity changes in a characteristic pattern when a particular organ is damaged, obstructed or failing. Clinical biochemistry interprets these patterns — hepatic, renal, cardiac, thyroid, pancreatic — to separate diseases that look alike at the bedside, such as pre-hepatic from obstructive jaundice. The topic carries heavy weight in B.Sc/M.Sc Biochemistry and Biotechnology papers, CSIR-UGC NET and GATE BT/XL (biochemistry and clinical sections), and in interviews for diagnostic laboratories (Dr Lal PathLabs, SRL, Metropolis, Thyrocare, Apollo, AIIMS/PGIMER labs) and for IVD kit makers (Transasia/Erba, Agappe, Mispa, Siemens, Roche India). On the job it governs analyser calibration, internal quality control and NABL (ISO 15189) accredited reporting under the Clinical Establishments Act, 2010.

Explained

Principles, specimens and reference intervals

• A function test measures either (a) a substance the organ clears (urea, creatinine, bilirubin), (b) a substance it synthesises (albumin, clotting factors, glucose), or (c) an enzyme that leaks out of damaged cells (ALT, CK-MB, troponin — strictly *damage* markers, not function markers).
• Reference interval: central 95% of values in a healthy reference population (mean ± 2 SD for Gaussian data), so 1 in 20 healthy people falls outside it. Must be age-, sex- and often population-specific; Indian adult creatinine and vitamin D intervals differ from Western kits, so NABL expects local verification of the manufacturer's range.
• Critical (panic) value: a result needing immediate telephonic alert, e.g. serum potassium above ~6.5 mmol/L or glucose below ~40 mg/dL (each lab fixes its own list).
• Specimen rules: plain/serum-separator tube for most chemistry; fluoride-oxalate (grey) for glucose (inhibits glycolysis); EDTA (lavender) for HbA1c; heparin (green) for ABG and ammonia on ice; citrate (blue) for PT/INR. Avoid haemolysis (falsely raises K⁺, LDH, AST) and prolonged tourniquet.
• Analytical quality: internal QC with two or three control levels plotted on a Levey–Jennings chart and judged by Westgard rules (1₃s, 2₂s, R₄s, 4₁s, 10x); external quality assurance through EQAS/PT schemes (CMC Vellore EQAS, AIIMS, RIQAS). Precision = CV; trueness = bias; total error must stay inside the lab's allowable limit.
• Units: India reports conventional units widely (mg/dL for glucose, creatinine, bilirubin; U/L for enzymes; mmol/L for electrolytes). SI equivalents are examinable — know the conversion factors.

Liver function tests (LFT)

• Bilirubin: haem catabolism gives unconjugated (indirect, lipid-soluble, albumin-bound) bilirubin; hepatocytes conjugate it with UDP-glucuronic acid by UGT1A1 to conjugated (direct, water-soluble) bilirubin excreted in bile. Measured by the van den Bergh / diazo reaction with sulphanilic acid: direct reaction without accelerator gives conjugated, total needs methanol/caffeine accelerator; indirect = total − direct.
• Jaundice appears clinically when total bilirubin exceeds about 2–3 mg/dL (scleral icterus). Classification:
  • *Pre-hepatic (haemolytic):* unconjugated predominates, urine bilirubin absent (it is albumin-bound, not filtered), urine/faecal urobilinogen high, LDH high, haptoglobin low, reticulocytes up.
  • *Hepatic (hepatocellular):* both fractions rise, ALT/AST markedly up, PT prolonged, albumin low in chronic disease.
  • *Post-hepatic (obstructive/cholestatic):* conjugated predominates, urine bilirubin present, pale stools, ALP and GGT markedly up, cholesterol up.
  • *Neonatal:* physiological jaundice from immature UGT1A1; Crigler–Najjar (UGT1A1 absent/deficient), Gilbert syndrome (mild reduced UGT1A1, benign), Dubin–Johnson (conjugated hyperbilirubinaemia from defective canalicular MRP2 export) and Rotor (conjugated hyperbilirubinaemia from impaired hepatic uptake/storage, OATP1B1/1B3 defect).
• Transaminases (aminotransferases): ALT (SGPT) is cytosolic and liver-specific; AST (SGOT) is cytosolic + mitochondrial and also in heart, muscle, kidney, RBC. Both need pyridoxal-5′-phosphate (vitamin B6) as coenzyme; assayed kinetically at 340 nm by NADH consumption (IFCC method).
  • De Ritis ratio = AST/ALT. <1 in viral hepatitis and NAFLD; >2 with high GGT in alcoholic liver disease (B6 deficiency lowers ALT more); >1 also in cirrhosis.
  • Values above ~1,000 U/L suggest acute viral hepatitis, paracetamol toxicity or ischaemic hepatitis.
• Cholestatic enzymes: ALP (membrane-bound, also bone, placenta, intestine; raised in obstruction, bone disease, pregnancy, growing children) — origin confirmed by GGT (liver/biliary, also induced by alcohol and phenytoin) or heat-stability/electrophoresis of isoenzymes. 5′-nucleotidase is liver-specific too.
• Synthetic function: albumin (half-life ~20 days, so falls in chronic disease; normal in acute hepatitis), total protein, A/G ratio (reverses in cirrhosis as globulins rise), prothrombin time/INR (factors II, VII, IX, X are vitamin-K dependent and short-lived — the earliest marker of synthetic failure).
• Detoxification: blood ammonia rises in hepatic encephalopathy (urea cycle failure and porto-systemic shunting).
• Dye-clearance and galactose-tolerance tests are of historical/theory interest; BSP is obsolete.

Renal function tests (RFT/KFT)

• GFR is the best overall index of renal function (~90–120 mL/min/1.73 m² in healthy adults). An ideal clearance marker is freely filtered, not reabsorbed, not secreted, not metabolised — inulin is the gold standard (research only); radio-labelled ⁵¹Cr-EDTA, iohexol are reference alternatives.
• Serum creatinine: from non-enzymatic breakdown of muscle creatine phosphate; production proportional to muscle mass, so low in women, children and the malnourished. Measured by the Jaffe reaction (alkaline picrate → orange-red Janovski complex, read at ~505 nm), which suffers pseudo-creatinine interference from glucose, ketones, ascorbate, bilirubin and cephalosporins; enzymatic (creatininase) methods are more specific. Creatinine is *insensitive early* — it rises only after GFR falls by roughly half (creatinine-blind range).
• Creatinine clearance needs a timed (24 h) urine; it overestimates GFR by ~10–15% because of tubular secretion of creatinine.
• eGFR equations: CKD-EPI 2009/2021 (recommended for adults; the 2021 version removed the race coefficient), MDRD, Cockcroft–Gault (creatinine clearance, body-weight based, used for drug dosing), Schwartz formula in children. An Indian-specific adjustment factor has been reported for MDRD; local validation is advised rather than assuming Western coefficients.
• Urea (BUN): end product of the urea cycle; a poorer marker than creatinine because it varies with protein intake, catabolism, GI bleeding and hydration, and is partly reabsorbed. BUN:creatinine ratio (mg/dL basis, normal ~10–20:1; a urea:creatinine ratio is about 2.14× higher) is raised in pre-renal azotaemia and GI bleed, normal/low in intrinsic renal disease. Note BUN = urea × 0.467 (urea nitrogen is 28/60 of urea).
• Newer markers: cystatin C (independent of muscle mass; used in combined eGFR equations), NGAL and KIM-1 for early acute kidney injury.
• Tubular function: urine specific gravity and osmolality (concentration test), fractional excretion of sodium (<1% pre-renal, >2% intrinsic ATN), urine acidification test for renal tubular acidosis, aminoaciduria/glycosuria in Fanconi syndrome.
• Proteinuria: dipstick detects albumin mainly; albumin:creatinine ratio (ACR) on an early-morning spot sample grades albuminuria (A1 <30, A2 30–300, A3 >300 mg/g). KDIGO CKD staging combines G1–G5 by eGFR with A1–A3 by ACR; dialysis is usually considered around eGFR <15 (G5).
• Acid–base and electrolytes: Na⁺, K⁺, Cl⁻, HCO₃⁻, calcium, phosphate, uric acid; renal failure gives high K⁺, high phosphate, low calcium, metabolic acidosis with raised anion gap.

Cardiac, pancreatic, muscle and gastric markers

• Cardiac troponin I/T — the reference standard for myocardial infarction: rises in 3–4 h, peaks 12–24 h, stays elevated 7–10 days; high-sensitivity troponin allows 0/1 h or 0/3 h rule-in/rule-out algorithms. A *rise and/or fall* with one value above the 99th percentile upper reference limit plus ischaemic evidence defines MI (Universal Definition).
• CK-MB — rises 4–6 h, peaks ~24 h, normalises in 48–72 h, so useful for *re-infarction*; CK-MB/total CK relative index above ~5% suggests cardiac origin. Total CK and CK-MM reflect skeletal muscle (rhabdomyolysis, myopathy, IM injection, exercise).
• Myoglobin is earliest (1–3 h) but non-specific; LDH (LDH1>LDH2 "flipped ratio") and AST are obsolete for MI.
• BNP / NT-proBNP for heart failure diagnosis and severity; lipid profile and hs-CRP for cardiovascular risk.
• Pancreas: amylase rises in 2–12 h and normalises in 3–5 days; lipase is more specific and stays up 8–14 days. Acute pancreatitis is suggested by levels above ~3 times the upper reference limit. Amylase also rises in mumps/parotitis, macroamylasaemia and renal failure. Exocrine insufficiency: faecal elastase, faecal fat.
• Stomach/intestine: gastrin, D-xylose absorption test, lactose tolerance/hydrogen breath test, faecal occult blood.
• Bone/other enzymes: acid phosphatase and PSA (prostate), ALP isoenzymes (bone), ACE (sarcoidosis).

Thyroid, adrenal and gonadal function

• Thyroid: TSH is the most sensitive first-line test because of the log-linear negative feedback (a small fall in free T4 produces a large rise in TSH).
  • *Primary hypothyroidism:* TSH high, free T4 low. *Subclinical hypothyroidism:* TSH high, free T4 normal.
  • *Primary hyperthyroidism (Graves):* TSH low, free T4/T3 high, TSH-receptor antibodies positive.
  • *Secondary/central:* TSH low or inappropriately normal with low free T4.
  • Total T3/T4 depend on thyroxine-binding globulin (raised in pregnancy and oestrogen use), so free hormone assays are preferred; measure anti-TPO for autoimmune thyroiditis. Neonatal TSH screening is recommended practice; India's iodised-salt programme (NIDDCP) targets adequate iodine, monitored by urinary iodine excretion.
• Adrenal cortex: morning serum cortisol, 24 h urinary free cortisol, late-night salivary cortisol; overnight 1 mg dexamethasone suppression test screens for Cushing syndrome (failure to suppress is positive); ACTH stimulation (Synacthen) test for Addison disease; aldosterone:renin ratio for primary hyperaldosteronism; plasma free metanephrines/urinary VMA for phaeochromocytoma.
• Pancreatic endocrine: fasting plasma glucose, 75 g OGTT, HbA1c (DCCT/NGSP-aligned, reported in % with IFCC mmol/mol), C-peptide to assess residual β-cell function; ICMR India guidelines use the standard WHO/ADA cut-offs — FPG ≥126 mg/dL, 2 h ≥200 mg/dL, HbA1c ≥6.5%.
• Gonadal/pituitary: LH, FSH, prolactin, testosterone, oestradiol, AMH; growth-hormone suppression (OGTT) for acromegaly and stimulation (clonidine/insulin tolerance) for deficiency.

Interpretation strategy, limitations and Indian regulatory frame

• Always read a pattern, not a single value: hepatocellular versus cholestatic injury, pre-renal versus renal azotaemia, primary versus central endocrine failure.
• Sensitivity = ability to detect disease (few false negatives, good for screening); specificity = ability to exclude it; predictive values depend on prevalence — a crucial point for mass screening in India where TB and diabetes prevalence differ by region.
• Pre-analytical errors cause the majority of laboratory errors: wrong tube, haemolysis, non-fasting sample, delayed separation (glucose falls ~5–7% per hour in a plain tube at room temperature), wrong patient identity.
• Drug and physiological interferences: phenytoin/alcohol raise GGT; statins raise CK and ALT; oral contraceptives raise TBG; pregnancy raises ALP and lowers creatinine; strenuous exercise raises CK and AST.
• Regulation and standards: laboratory accreditation by NABL under ISO 15189 (medical laboratories) and ISO 15190 for safety; registration under the Clinical Establishments (Registration and Regulation) Act, 2010 in adopting states; devices and kits under the Medical Devices Rules, 2017 framed under the Drugs and Cosmetics Act, 1940, with CDSCO licensing (IVDs are notified medical devices, risk-classed A–D); biomedical waste segregation under the Bio-Medical Waste Management Rules, 2016 (as amended) — yellow, red, white and blue bins; the Pre-conception and Pre-natal Diagnostic Techniques (PCPNDT) Act, 1994 governs prenatal diagnostic work. Workplace safety now sits under the Occupational Safety, Health and Working Conditions Code, 2020, in force from 21 November 2025. Specific fee limits and state rules vary — check the current notification rather than assuming.

Key points

• Function vs damage marker - albumin and PT measure liver function; ALT and troponin measure cell damage.
• Reference interval - central 95% of healthy values; 5% of healthy people lie outside by definition.
• Van den Bergh reaction - direct (no accelerator) = conjugated bilirubin; total needs an accelerator; indirect = total − direct.
• Urine bilirubin - present in obstructive/hepatic jaundice, absent in haemolytic jaundice (unconjugated bilirubin is albumin-bound).
• ALT is liver-specific, AST is not - AST also comes from heart, skeletal muscle and RBC.
• De Ritis ratio AST/ALT >2 - alcoholic liver disease; <1 - viral hepatitis or NAFLD.
• ALP + GGT both high - cholestasis; ALP high with normal GGT - bone origin.
• PT/INR prolongation is the earliest sign of failing hepatic synthesis (short half-life clotting factors).
• GFR is the best single index of renal function; inulin clearance is the gold standard, eGFR the practical one.
• Creatinine-blind range - serum creatinine stays normal until GFR falls by about half.
• Jaffe reaction - alkaline picrate method for creatinine; prone to pseudo-creatinine interference; enzymatic assays are more specific.
• BUN:creatinine ratio >20:1 (mg/dL basis) suggests pre-renal azotaemia or GI bleeding.
• Cystatin C - GFR marker independent of muscle mass, useful in cachexia and children.
• ACR + eGFR - the two axes of KDIGO CKD staging (G1–G5, A1–A3).
• Troponin I/T - most sensitive and specific MI marker (3–4 h to 7–10 days); CK-MB detects re-infarction.
• Lipase beats amylase for pancreatitis specificity and stays raised longer.
• TSH first - log-linear feedback makes it the most sensitive thyroid test; free T4 defines the level of the lesion.
• Westgard rules on Levey–Jennings charts control analytical quality; EQAS verifies accuracy against peers.
• NABL (ISO 15189) accredits Indian medical labs; IVD kits are regulated by CDSCO under the Medical Devices Rules, 2017.

Formulas and examples

Example 1 — Jaundice classification. Total bilirubin 9.0 mg/dL, direct 7.2 mg/dL, ALP 420 U/L, ALT 70 U/L, GGT 310 U/L, urine bilirubin positive.

• Indirect = 9.0 − 7.2 = 1.8 mg/dL; conjugated share = 7.2/9.0 = 80%.
• Conjugated predominance + markedly raised ALP and GGT + bilirubin in urine, with only mildly raised ALT → post-hepatic (obstructive/cholestatic) jaundice.
• In SI units total bilirubin = 9.0 × 17.1 ≈ 154 µmol/L.

Example 2 — Creatinine clearance from a 24 h urine. 24 h urine volume 1,440 mL, urine creatinine 90 mg/dL, serum creatinine 1.5 mg/dL, BSA 1.60 m².

• V = 1,440 mL / 1,440 min = 1.0 mL/min.
• C = (90 × 1.0) / 1.5 = 60 mL/min.
• Corrected = 60 × 1.73/1.60 = 64.9 mL/min/1.73 m² → CKD stage G2 (60–89, mildly decreased), close to the G3a boundary (45–59); ACR is needed to complete staging.

Example 3 — Cockcroft–Gault for drug dosing. Woman, 62 years, 55 kg, serum creatinine 1.8 mg/dL.

• Numerator = (140 − 62) × 55 × 0.85 = 78 × 55 × 0.85 = 4,290 × 0.85 = 3,646.5.
• Denominator = 72 × 1.8 = 129.6.
• CrCl = 3,646.5 / 129.6 ≈ 28.1 mL/min → reduce renally cleared drug doses (e.g. many antibiotics) substantially.

Example 4 — Urea:creatinine ratio and azotaemia. BUN 42 mg/dL, serum creatinine 1.4 mg/dL, patient dehydrated after vomiting.

• Ratio = 42/1.4 = 30:1 (normal 10–20:1) → pre-renal azotaemia, consistent with volume depletion.
• Urea = 42 × 2.14 ≈ 90 mg/dL (if the lab reports urea, not BUN); urea in SI = 90 × 0.1665 ≈ 15.0 mmol/L.

Example 5 — Anion gap and corrected calcium in renal failure. Na⁺ 134, Cl⁻ 98, HCO₃⁻ 12 mmol/L; measured calcium 7.2 mg/dL with albumin 2.5 g/dL.

• Anion gap = 134 − (98 + 12) = 24 mmol/L → high anion gap metabolic acidosis (retained sulphate, phosphate, organic acids in uraemia).
• Corrected calcium = 7.2 + 0.8 × (4.0 − 2.5) = 7.2 + 1.2 = 8.4 mg/dL → the apparent hypocalcaemia is largely due to low albumin.

Where it is used

• Diagnostic laboratory chains and hospital labs (Dr Lal PathLabs, SRL, Metropolis, Thyrocare, Apollo, Max, AIIMS/CMC/PGIMER): roles as clinical biochemist, lab technologist, QC officer, NABL quality manager preparing ISO 15189 documents and EQAS submissions.
• IVD reagent and analyser companies (Transasia/Erba, Agappe Diagnostics, Mispa, Siemens Healthineers, Roche, Beckman, Tulip): kit development, method validation, linearity and interference studies, application and service support for autoanalysers.
• Pharmaceutical and CRO clinical trials (Syngene, Lambda, Veeda, Sun, Cipla, Dr Reddy's): LFT/RFT/cardiac panels as safety endpoints for hepatotoxicity, nephrotoxicity and cardiotoxicity under GLP/GCP and CDSCO's New Drugs and Clinical Trials Rules, 2019.
• Public health screening programmes: NIDDCP (thyroid/iodine), NP-NCD screening for diabetes and hypertension at HWCs, neonatal screening programmes in some states, Ayushman Bharat PM-JAY diagnostic packages.
• Point-of-care and home diagnostics: glucometers, HbA1c and troponin POCT, dry-chemistry analysers used in PHCs and ambulances; teleconsultation-linked home collection logistics.
• Veterinary and food/nutraceutical labs apply the same enzyme and clearance chemistry; forensic and toxicology labs use LFT/RFT patterns in poisoning cases.

Interview questions

• Q: Why is ALT more liver-specific than AST? A: ALT is predominantly cytosolic and concentrated in hepatocytes, whereas AST is present in heart, skeletal muscle, kidney and red cells as well as liver, so AST rises in non-hepatic injury too.
• Q: Why is urine bilirubin absent in haemolytic jaundice? A: The excess is unconjugated bilirubin, which is tightly bound to albumin and therefore not filtered by the glomerulus; only water-soluble conjugated bilirubin appears in urine.
• Q: Why does serum creatinine miss early renal disease? A: Because of renal reserve and hyperfiltration by surviving nephrons, creatinine stays within the reference interval until GFR falls by about 50% — the creatinine-blind range.
• Q: Why is creatinine preferred over urea as a renal marker? A: Creatinine production is relatively constant with muscle mass and it is not reabsorbed, while urea varies with protein intake, catabolism and hydration and is partly reabsorbed.
• Q: What makes inulin the ideal GFR marker? A: It is freely filtered and neither reabsorbed, secreted nor metabolised, so its clearance equals GFR exactly — but it needs infusion and timed sampling, so it is used only in research.
• Q: Which single test do you order first for suspected thyroid disease and why? A: TSH, because the pituitary responds log-linearly to small changes in free T4, making TSH abnormal before T4 leaves the reference interval.
• Q: Troponin is already the best MI marker — why still measure CK-MB? A: CK-MB clears in 48–72 h while troponin stays raised for up to 10 days, so a fresh CK-MB rise is useful for detecting re-infarction; it also helps when troponin assays are unavailable.
• Q: Patient has ALP 600 U/L — how will you find out whether it is liver or bone? A: Measure GGT or 5′-nucleotidase: both raised means hepatobiliary, a normal GGT with high ALP points to bone (or pregnancy/placental origin); isoenzyme electrophoresis or heat-stability confirms.
• Q: What is the De Ritis ratio and its clinical use? A: The AST/ALT ratio; greater than 2 (usually with raised GGT and macrocytosis) suggests alcoholic liver disease, below 1 suggests viral hepatitis or fatty liver, and it rises again in cirrhosis.
• Q: Which interferences affect the Jaffe creatinine method? A: Glucose, ketone bodies, ascorbate, bilirubin, protein and some cephalosporins act as pseudo-creatinine chromogens; enzymatic creatininase methods avoid most of them.
• Q: How do you decide a run is out of control? A: Plot controls on a Levey–Jennings chart and apply Westgard rules — 1₃s or 2₂s indicates random or systematic error respectively and the run is rejected, re-calibrated and repeated.
• Q: Which Indian accreditation and regulation apply to a clinical biochemistry laboratory? A: NABL accreditation to ISO 15189, registration under the Clinical Establishments Act, 2010 in adopting states, IVD kits licensed by CDSCO under the Medical Devices Rules, 2017, and waste handled under the Bio-Medical Waste Management Rules, 2016.
• Q: Why is lipase preferred to amylase in acute pancreatitis? A: Lipase is more pancreas-specific and remains elevated for 8–14 days, while amylase normalises in 3–5 days and also rises in parotitis, renal failure and macroamylasaemia.

Common mistakes

• Calling conjugated bilirubin "indirect" — the direct-reacting fraction is the conjugated one; indirect is calculated as total minus direct.
• Treating raised enzymes as a measure of function; a cirrhotic liver can have near-normal ALT with a grossly abnormal INR and albumin.
• Writing BUN values as urea, or converting with the wrong factor (urea = BUN × 2.14, BUN = urea × 0.467).
• Forgetting to normalise clearance to 1.73 m² body surface area, or forgetting that V must be in mL/min (24 h volume ÷ 1,440).
• Using serum creatinine alone to declare normal kidneys, and ignoring albuminuria (ACR), which can be abnormal at a normal eGFR.
• Ignoring pre-analytical causes — haemolysis (falsely high K⁺, AST, LDH), a plain tube for glucose, a non-fasting lipid sample — and reporting the artefact as disease.
• Quoting a kit's Western reference interval without local verification, and failing to correct calcium for a low albumin.

Related topics

Enzymes: classification, kinetics and inhibition · Nitrogen metabolism and the urea cycle · Haem synthesis and degradation · Endocrinology: hormones and signal transduction · Biochemical techniques: spectrophotometry and electrophoresis · Carbohydrate metabolism and diabetes mellitus