Kidney Function Tests
On this page
Direct answer
Muscle makes creatinine at a near-constant rate from creatine phosphate, and that predictability is what makes it the workhorse of renal testing — freely filtered, barely reabsorbed, independent of hydration — with adult references of about 0.7-1.3 mg/dL in men and 0.6-1.1 mg/dL in women (Jaffe's alkaline picrate method on most Indian benches). Urea (15-45 mg/dL) is the junior partner precisely because it is fickle: reabsorbed variably with hydration and shifted by diet, GI bleeding and catabolism. Because creatinine alone hides substantial function loss, estimating equations convert it into glomerular filtration rate — Cockcroft-Gault for drug dosing, MDRD and CKD-EPI for chronic kidney disease staging from G1 (normal or high GFR) to G5 (below 15 mL/min/1.73 m²).
What you must remember
- Creatinine measurement: Jaffe's reaction — creatinine plus picric acid in alkali gives an orange-red colour; non-creatinine chromogens slightly overestimate, and enzymatic assays are more specific.
- Urea rises disproportionately over creatinine in dehydration (pre-renal), GI bleed, high protein load, catabolism and steroids; the blood urea nitrogen-to-creatinine ratio (normal 10-20:1) exceeding 20 suggests a pre-renal picture.
- Cockcroft-Gault: creatinine clearance = (140 − age) × weight (kg) ÷ (72 × serum creatinine in mg/dL), multiplied by 0.85 for women — still the reference for drug dosing such as vancomycin.
- CKD G-categories: G1 ≥90, G2 60-89, G3 30-59, G4 15-29, G5 below 15 mL/min/1.73 m²; albuminuria categories A1-A3 by urine albumin-to-creatinine ratio (under 30, 30-300, over 300 mg/g).
- A modest creatinine rise is significant: because the creatinine-versus-GFR curve is rectangular, doubling of creatinine can mean halving of filtration.
- Twenty-four-hour urine collections (protein, creatinine clearance) fail silently when collection is incomplete — the spot ACR has largely replaced them for proteinuria staging.
- Tubular function checks complement the glomerular panel: fixed specific gravity (isosthenuria), urine concentrating ability and fractional excretion of sodium in acute kidney injury workups.
A rising creatinine, reasoned through
A 70-year-old woman, three days after hip surgery, has creatinine 0.9 rising to 1.6 mg/dL with urine output fallen to 300 mL/day. The structured question is prerenal, renal or postrenal. Prerenal is assessed first because it is commonest and reversible: she was nil by mouth, has tachycardia, and her urea-to-creatinine ratio has climbed to 25 — favours prerenal perfusion failure. Intrinsic renal (acute tubular necrosis) argues its case with muddy-brown granular casts in the sediment and a urine sodium and fractional excretion above 2 per cent (a tubule that cannot reabsorb sodium); prerenal oliguria typically shows urine sodium under 20 and concentrated urine. Postrenal is excluded with a bladder scan or ultrasound — no hydronephrosis, no retention.
Fluid challenge settles it: output rises and creatinine drifts to 1.1 over two days, confirming perfusion. Contrast this with the parallel bed's long-standing type 2 diabetic whose creatinine is stable at 1.8 with an eGFR of 38 (G3b) and ACR 400 mg/g (A3) — not an emergency but a trajectory, and the response is renoprotective therapy, glycaemic and blood pressure control, and avoidance of nephrotoxins. Same analyte, two completely different interpretations — the number means nothing without its curve and its context.
Where students slip
"Creatinine normal, kidneys fine" is the reflex that fails: the creatinine plot against GFR is hyperbolic: a substantial fraction of function is lost before creatinine leaves the reference interval, so trend matters more than any single value. The mirror slip treats one number as universal — a muscular young athlete at 1.3 has a healthy mass of muscle, while a frail elderly woman at 1.2 may already be in stage 3 CKD; the estimating equations exist precisely because age, sex and weight bend the interpretation. Finally, attributing a raised urea alone to renal failure ignores the dehydration, GI bleed and steroid threads that raise urea without touching the kidney.
Frequently asked questions
What are normal serum creatinine values and why do they differ by sex?
About 0.7-1.3 mg/dL in men and 0.6-1.1 in women, reflecting muscle mass, the source of creatinine.
What does a urea-to-creatinine ratio above 20 suggest?
A pre-renal state such as dehydration or hypovolaemia, or increased urea generation (GI bleed, steroids, high protein intake).
Write the Cockcroft-Gault formula and state its use.
(140 − age) × weight(kg) ÷ (72 × serum creatinine mg/dL), ×0.85 for women; used mainly for dosing renally cleared drugs.
How is chronic kidney disease staged?
By estimated GFR: G1 ≥90, G2 60-89, G3 30-59, G4 15-29, G5 below 15 mL/min/1.73 m², cross-classified with albuminuria categories A1-A3.
What is the principle and limitation of the Jaffe method?
Alkaline picrate forms an orange-red chromogen with creatinine; non-creatinine chromogens (glucose, ketones) interfere, mildly overestimating true creatinine.
Which urine test quantifies proteinuria in CKD?
The spot urine albumin-to-creatinine ratio, with A2 (30-300 mg/g) moderately increased and A3 (over 300 mg/g) severely increased albuminuria.