Kidney Function Tests (KFT)

KFT (also called Renal Function Tests / RFT) is a panel of blood and urine investigations used to evaluate how well the kidneys are filtering blood, concentrating urine, and maintaining homeostasis.

1. Urine-Based Tests

Urine Specific Gravity

Normal: 1.010-1.025
With diminishing renal function, concentrating ability is progressively lost until specific gravity fixes at 1.006-1.010 (isosthenuria)
Diluting ability is maintained longer; even in uremia, dilution to 1.002-1.004 may persist
Urine osmolality is a more precise measure but specific gravity is convenient for office use

Urine Protein / Albumin

Normal proteinuria: < 150 mg/day (mainly Tamm-Horsfall protein)
Urine dipstick is most sensitive to albumin; less sensitive to other proteins
Microalbumin assay detects as little as 0.3 mg/dL - a sensitive early marker of glomerular dysfunction
Urine β2-microglobulin and lysozyme detect tubular dysfunction
Significant proteinuria is quantified by 24-hour urine collection

2. Blood/Serum Tests

Serum Creatinine

Creatinine is the end product of creatine metabolism in skeletal muscle, excreted by the kidneys
Normal: 0.8-1.2 mg/dL (adults); 0.4-0.8 mg/dL (young children)
Serum creatinine stays within normal range until approximately 50% of renal function is lost
Not significantly influenced by dietary intake (unlike BUN)
Measured classically by the Jaffe reaction (alkaline picrate forms colored complex)
- Falsely elevated by: cephalosporins, ketones, glucose, fructose, protein, urea, ascorbic acid

Blood Urea Nitrogen (BUN)

Urea is the primary metabolite of protein catabolism, excreted entirely by the kidneys
BUN is influenced by: dietary protein intake, hydration status, GI bleeding
Approximately two-thirds of renal function must be lost before BUN significantly rises
An increase in BUN = azotemia; high BUN with toxic effects = uremia
Normal: approximately 7-20 mg/dL

BUN:Creatinine Ratio

Normal: ~10:1
Elevated (20:1 to 40:1): prerenal azotemia (dehydration, bilateral obstruction, urinary extravasation)
Low or normal: intrarenal disease, hepatic insufficiency, overhydration

3. Glomerular Filtration Rate (GFR)

Endogenous Creatinine Clearance

Most accurate standard measure without exogenous infusion
Uses 24-hour urine collection + concurrent serum specimen
Normal: 90-110 mL/min (70-140 mL/min after correction for body surface area)
Formula: ClCr = (UCr × V) / (PCr × time)

Estimated GFR (eGFR) Equations

Equation	Notes
Cockcroft-Gault	Classic formula, uses creatinine + age + weight + sex
MDRD	Widely used; validated in adults (Caucasian/African American) with GFR < 60; NOT reliable for GFR > 60 or in pregnancy/children
CKD-EPI	Most accurate; reliable even when GFR > 60; best predictor of ESRD, cardiovascular, and all-cause mortality

4. Cystatin C

Produced by nearly all nucleated cells; freely filtered by glomerulus and completely reabsorbed by the proximal tubule
Superior to creatinine as a predictor of cardiovascular mortality
An early indicator of evolving chronic kidney disease
Increasingly used to calculate eGFR (independent of muscle mass)

5. Differentiating Acute Renal Failure (ARF) Types

Parameter	Prerenal ARF	Renal ARF
BUN:Creatinine ratio	> 20:1	< 20:1
Urine specific gravity	High (> 1.020)	Low (< 1.010)
Urine osmolality	High (> 500 mOsm/kg)	Low (300-400)
Fractional excretion of Na (FENa)	< 1%	> 2%
Fractional excretion of urea (FEUrea)	< 35%	> 35%

6. CKD Screening Recommendations

The National Kidney Foundation recommends annual testing for high-risk individuals (diabetes, hypertension, family history of renal disease), including:

eGFR
Microalbuminuria test

CKD is defined by:

GFR < 60 mL/min/1.73 m² or
Albuminuria persisting for 3 or more consecutive months

Sources: Smith and Tanagho's General Urology, 19th Ed. | Quick Compendium of Clinical Pathology, 5th Ed.

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