# Kidney Function Tests

> Kidney function tests for Medical Laboratory Technology: urea and creatinine, BUN ratio, Cockcroft-Gault and eGFR, CKD staging and urine protein checks.

- Canonical URL: https://prepelephant.com/topics/allied/medical-laboratory-technology/kidney-function-tests-lab
- Exam / course: Allied Health · Subject: Medical Laboratory Technology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Kidney Function Tests", PrepElephant, https://prepelephant.com/topics/allied/medical-laboratory-technology/kidney-function-tests-lab

## 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.