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Kidney Function Tests (KFTs) help evaluate glomerular filtration, tubular function, and electrolyte balance. Correct interpretation is essential in diagnosing acute kidney injury (AKI), chronic kidney disease (CKD), electrolyte imbalances, and acid-base disorders.
🔹 1. Key Components of KFT
| Test | Normal Range | Clinical Significance |
|---|---|---|
| Serum Creatinine (S.Cr) | 0.6 – 1.2 mg/dL | High → Kidney dysfunction |
| Blood Urea Nitrogen (BUN) | 7 – 20 mg/dL | High → Kidney dysfunction, dehydration |
| BUN/Creatinine Ratio | 10:1 to 20:1 | High → Pre-renal AKI, Low → Intrinsic AKI |
| Glomerular Filtration Rate (GFR) | >90 mL/min/1.73m² | Low → CKD progression |
| Serum Uric Acid | 3.5 – 7.2 mg/dL | High → Gout, CKD |
| Serum Sodium (Na⁺) | 135 – 145 mEq/L | Low → Hyponatremia, High → Hypernatremia |
| Serum Potassium (K⁺) | 3.5 – 5.0 mEq/L | High → AKI, CKD, Addison’s |
| Serum Chloride (Cl⁻) | 96 – 106 mEq/L | High → Acidosis, Low → Alkalosis |
| Serum Bicarbonate (HCO₃⁻) | 22 – 28 mEq/L | Low → Metabolic acidosis, High → Metabolic alkalosis |
🔹 2. Interpretation Based on Patterns
A. Acute Kidney Injury (AKI)
Defined as a rapid decline in kidney function (within hours to days).
📌 Key Lab Findings:
- ↑ Creatinine (>0.3 mg/dL in 48 hrs OR >50% increase from baseline)
- ↓ GFR
- ↓ Urine Output (<0.5 mL/kg/hr for 6 hours)
AKI Classification (KDIGO Criteria):
| Stage | Creatinine Increase | Urine Output |
|---|---|---|
| Stage 1 | 1.5 – 1.9x baseline OR ↑ ≥0.3 mg/dL | <0.5 mL/kg/hr for 6 hrs |
| Stage 2 | 2.0 – 2.9x baseline | <0.5 mL/kg/hr for 12 hrs |
| Stage 3 | >3x baseline OR ≥4 mg/dL | <0.3 mL/kg/hr for 24 hrs OR Anuria |
B. Types of AKI
| Type | BUN/Creatinine Ratio | Urine Sodium (UNa) | FENa (%): Fractional Excretion of Na⁺ | Urinalysis |
|---|---|---|---|---|
| Pre-Renal (Hypovolemia, CHF, Sepsis, Liver Cirrhosis) | >20:1 | <20 mEq/L | <1% | Bland Sediment |
| Intrinsic (ATN, Glomerulonephritis, Interstitial Nephritis) | 10-15:1 | >40 mEq/L | >2% | Muddy brown casts (ATN), RBC casts (GN), WBC casts (AIN) |
| Post-Renal (Obstruction: Stones, BPH, Tumor) | Variable | Variable | Variable | Variable Sediment |
📌 Example Interpretation:
- BUN/Creatinine = 25:1, Urine Na <10 → Pre-Renal AKI (Dehydration, Heart Failure).
- BUN/Creatinine = 12:1, Urine Na >40 → Intrinsic AKI (Acute Tubular Necrosis).
C. Chronic Kidney Disease (CKD)
Defined as GFR <60 mL/min for >3 months OR Kidney Damage (Proteinuria, Abnormal Imaging).
📌 Key Lab Findings:
- ↓ GFR (<90)
- ↑ Creatinine & BUN
- Proteinuria (Albumin/Creatinine Ratio >30 mg/g)
- Electrolyte Imbalances (Hyperkalemia, Metabolic Acidosis)
CKD Stages (KDIGO):
| Stage | GFR (mL/min/1.73m²) | Clinical Significance |
|---|---|---|
| Stage 1 | >90 (with evidence of kidney damage) | Early kidney damage (Proteinuria) |
| Stage 2 | 60 – 89 | Mild CKD |
| Stage 3a | 45 – 59 | Moderate CKD |
| Stage 3b | 30 – 44 | Moderate-severe CKD |
| Stage 4 | 15 – 29 | Severe CKD |
| Stage 5 (ESRD) | <15 | End-stage renal disease, Dialysis needed |
📌 Example Interpretation:
- GFR = 40, Proteinuria = 500 mg/day → CKD Stage 3b.
D. Electrolyte & Acid-Base Disturbances in Kidney Disease
| Disorder | Lab Findings | Clinical Conditions |
|---|---|---|
| Hyponatremia (↓ Na⁺) | Na⁺ <135 | SIADH, CKD, CHF, Cirrhosis |
| Hypernatremia (↑ Na⁺) | Na⁺ >145 | Dehydration, DI |
| Hypokalemia (↓ K⁺) | K⁺ <3.5 | Diuretics, RTA, Vomiting |
| Hyperkalemia (↑ K⁺) | K⁺ >5.5 | AKI, CKD, Addison’s, Rhabdo |
| Metabolic Acidosis | HCO₃⁻ <22, pH <7.35 | AKI, CKD, DKA |
| Metabolic Alkalosis | HCO₃⁻ >28, pH >7.45 | Vomiting, Diuretics |
📌 Example Interpretation:
- Na⁺ = 125, Serum Osmolality = 260 → SIADH.
- K⁺ = 6.2, ECG = Peaked T waves → Hyperkalemia (urgent treatment needed).
🔹 3. Interpretation Summary
| Lab Pattern | Likely Causes |
|---|---|
| Creatinine ↑ + BUN/Cr > 20:1 | Pre-Renal AKI (Dehydration, CHF, Cirrhosis) |
| Creatinine ↑ + BUN/Cr ~ 10-15:1 + Muddy Casts | Intrinsic AKI (Acute Tubular Necrosis) |
| Creatinine ↑ + Obstruction on Imaging | Post-Renal AKI (Kidney Stones, BPH) |
| GFR <60 for >3 months + Proteinuria | Chronic Kidney Disease (CKD) |
| Hyperkalemia + Metabolic Acidosis | Renal Failure, Addison’s |
| Hyponatremia + Low Serum Osmolality | SIADH, CKD |
🔹 4. Clinical Decision-Making
🔹 If Creatinine is suddenly elevated?
→ Check Urine Na & BUN/Cr ratio to differentiate Pre-Renal vs. ATN.
🔹 If CKD is diagnosed?
→ Monitor GFR, Proteinuria, Electrolytes regularly.
🔹 If Potassium is >6.5?
→ Immediate intervention (Calcium Gluconate, Insulin, Dialysis).