Calculators and Tools for Chronic Kidney Disease

CKD staging by eGFR: Stage 1 = eGFR ≥90 (with kidney damage markers); Stage 2 = 60–89; Stage 3a = 45–59; Stage 3b = 30–44; Stage 4 = 15–29; Stage 5 = <15 (kidney failure). KDIGO recommends nephrology referral for: eGFR <30 (Stage 4+), rapid decline (>5 mL/min/year), urine ACR >300 mg/g (A3 category), CKD of uncertain etiology, refractory hypertension, or recurrent electrolyte abnormalities. Early nephrology involvement at Stage 3b improves preparation for renal replacement therapy and transplant listing.

Use CKD-EPI 2021 eGFR for: CKD staging, disease monitoring, and determining nephrology referral thresholds. Use Cockcroft-Gault creatinine clearance for: drug dosing decisions — virtually all pharmacokinetic studies that established dosing thresholds used CrCl. The two can differ significantly at extremes of weight and age. For obese patients, use adjusted body weight in Cockcroft-Gault to avoid overestimating renal function and potentially over-dosing renally-cleared drugs.

CKD medication adjustments by eGFR: <60 — begin monitoring renally-cleared drugs; <45 — reduce metformin dose, stop contrast-related procedures without hydration; <30 — stop metformin, nitrofurantoin (ineffective and toxic), avoid NSAIDs, adjust gabapentin significantly; <25 — avoid spironolactone; <15 — dialysis-specific dosing for most renally-cleared drugs. Always reference current drug monographs — guidance evolves. SGLT2 inhibitors (empagliflozin, dapagliflozin) have slowed CKD progression and are now recommended down to eGFR 20 for kidney protection.

Metabolic acidosis with elevated anion gap (>12 mEq/L) in CKD may reflect uremic acidosis (accumulation of sulfates, phosphates, and hippurate). However, always check for superimposed causes: lactic acidosis, ketoacidosis, or toxic ingestion. Correct the anion gap for hypoalbuminemia (add 2.5 mEq/L per 1 g/dL drop below 4 g/dL — common in CKD). Non-anion gap metabolic acidosis in CKD (due to reduced ammonia excretion) is treated with oral sodium bicarbonate supplementation — KDIGO recommends maintaining serum bicarbonate ≥22 mEq/L to slow CKD progression.

CKD causes hypoalbuminemia, which lowers total serum calcium without affecting ionized (physiologically active) calcium. The Payne correction adjusts for this: corrected calcium = measured calcium + 0.8 × (4 − albumin). Hyperphosphatemia and reduced renal activation of vitamin D in CKD cause secondary hyperparathyroidism, further dysregulating calcium-phosphorus balance. Monitor calcium, phosphate, PTH, and 25-OH vitamin D in Stage 3b+ CKD. Calcium-based phosphate binders should be used cautiously to avoid hypercalcemia and vascular calcification.

Fractional Excretion of Sodium (FENa) = (urine Na × plasma creatinine) ÷ (plasma Na × urine creatinine) × 100. FENa <1% suggests prerenal azotemia (kidneys avidly retaining sodium); FENa >2% suggests intrinsic renal injury (ATN). Important caveats: FENa <1% can occur in contrast nephropathy, myoglobinuria, and early obstructive uropathy. FENa is unreliable in patients on diuretics (use FEUrea instead: <35% = prerenal). Always interpret with urine output, clinical context, and urine microscopy.