Kidney Function Tests ,Values And Interpretation

Kidney Function Tests (KFTs) are a panel of biochemical tests used to evaluate renal function, essential in diagnosing and monitoring kidney diseases and conditions that affect renal function.

Overview of Kidney Physiology

The kidneys play critical roles in:

Excretion of nitrogenous waste products (e.g., urea, creatinine, uric acid).
Regulation of extracellular fluid volume, serum osmolality, and electrolyte balance.
Endocrine functions , including production of:
- Erythropoietin (stimulates RBC production),
- 1,25-dihydroxyvitamin D (active vitamin D for calcium metabolism),
- Renin (blood pressure regulation via RAAS).

Functional Unit: Nephron

Each nephron comprises:

Glomerulus : Filters plasma based on size and charge; negatively charged basement membrane restricts protein passage.
Tubules : Reabsorb water, electrolytes, nutrients, and secrete additional waste. Responsible for urine concentration.

Categories of Kidney Function Tests

Tests of Glomerular Function
Tests of Tubular Function

Panel of Kidney Function Tests

1. Urinalysis

Provides insight into renal pathology:

Physical properties : color, odor, specific gravity, volume.
Microscopy : RBCs, WBCs, casts, crystals.
Proteinuria : Marker of glomerular damage (e.g., nephrotic syndrome).

2. Serum Urea

Byproduct of protein metabolism (urea cycle in liver).
Filtered, reabsorbed, and secreted by nephrons.
Normal: 20–45 mg/dL
Increased: Renal dysfunction (↓GFR <50%), high protein diet, catabolic states.
Decreased: Liver disease, malnutrition.

Limitations : Urea levels are influenced by non-renal factors and rise late in kidney disease.

3. Blood Urea Nitrogen (BUN)

Reflects nitrogen component of urea.
Conversion: BUN = Urea × 0.47
Normal: 5–26 mg/dL
Increased in :
- Prerenal causes: Dehydration, CHF, GI bleeding
- Intrinsic renal disease
- Postrenal obstruction
Decreased in : SIADH, liver failure, malnutrition

Azotemia = ↑BUN and/or ↑serum creatinine

4. BUN/Creatinine Ratio

Normal: 10:1 to 20:1
Increased: Prerenal azotemia (e.g., dehydration, GI bleed)
Decreased: Liver disease, low protein intake, rhabdomyolysis

5. Serum Creatinine

Product of muscle metabolism.
Freely filtered , minimally secreted; not reabsorbed.
Normal:
- Males: 0.7–1.3 mg/dL
- Females: 0.6–1.1 mg/dL
More reliable than BUN for assessing GFR.
Increased: Decreased GFR
Decreased: Low muscle mass, pregnancy
Affected by medications (e.g., cimetidine, trimethoprim)

Measured by the Jaffe method in labs.

6. Creatinine Clearance (CrCl)

Estimates GFR:

Formula: CrCl = (U × V) / P
- U = urine creatinine (mg/dL)
- V = urine volume (mL/min)
- P = plasma creatinine (mg/dL)
Requires 24-hour urine and concurrent blood sample.
Normal:
- Males: 100–120 mL/min
- Females: 95–105 mL/min

7. Urea Clearance

Less accurate than CrCl due to tubular reabsorption of urea.
Standard clearance: ~54 mL/min
Max clearance: ~75 mL/min

8. Electrolytes

Sodium, Potassium, Calcium, Phosphate
Abnormalities may reflect impaired tubular reabsorption/secretion.

9. Serum Phosphorus

Normally excreted by kidneys.
Increased in : CKD, hypoparathyroidism
Complication : Calcium-phosphate deposition in soft tissues

10. Calcium

Altered in CKD due to disrupted vitamin D metabolism and secondary hyperparathyroidism.

11. Protein and Albumin

Proteinuria is a sensitive marker for renal disease.
- Nephrotic syndrome : >3–4 g/day
- Nephritic syndrome : 1–2 g/day
- Tubulointerstitial disease : <1 g/day
Hypoalbuminemia is common in nephrotic syndrome.