Monitoring Kidney Health: Tests and When to See a Doctor
Key kidney health tests: (1) Serum creatinine and estimated GFR (eGFR) — creatinine is a waste product of muscle metabolism filtered by the kidneys; the eGFR (calculated from creatinine, age, sex, and race) estimates the percentage of normal kidney filtering capacity remaining. Normal eGFR is above 90 mL/min/1.73m²; CKD stages are G1 (≥90, with evidence of damage), G2 (60-89), G3a (45-59), G3b (30-44), G4 (15-29), G5 (<15, kidney failure). The eGFR should be trended over time — a stable eGFR of 55 is far less concerning than one that has declined from 85 to 55 over 2 years.
(2) Urine albumin-to-creatinine ratio (UACR) — measures albumin (protein) leakage into urine. Normal is below 30 mg/g; microalbuminuria is 30-300 mg/g (early diabetic nephropathy); macroalbuminuria is above 300 mg/g. Proteinuria is an independent predictor of cardiovascular disease (not just kidney disease) and should always be investigated and treated. (3) Blood urea nitrogen (BUN) — rises with reduced GFR and with high protein intake or dehydration; less specific than creatinine but useful in context. (4) Serum electrolytes — potassium, bicarbonate, phosphate, and uric acid all reflect kidney function and are routinely tested in comprehensive metabolic panels.
Warning signs that warrant urgent evaluation: foamy, dark, or tea-colored urine (proteinuria or hematuria); significantly reduced urine output; swelling in the legs or feet (edema from impaired fluid regulation); fatigue and loss of appetite (uremia from reduced toxin clearance); flank pain (acute kidney injury, kidney stones, or infection); persistent itching (uremia); difficulty concentrating or “brain fog” (uremic encephalopathy in advanced CKD). Any of these symptoms should prompt same-day or next-day evaluation rather than waiting for scheduled appointments.
Dietary considerations in CKD: protein restriction (0.6-0.8g/kg/day under nephrology supervision) slows CKD progression in stages 3-5 by reducing the filtration burden and decreasing the production of urea and other nitrogenous waste products — however, protein restriction must be balanced against the risk of malnutrition, particularly in older adults and those with low albumin. Phosphorus restriction (limiting dairy, processed foods, dark colas) becomes important in stages 3-5 as the kidneys lose capacity to excrete phosphorus, leading to secondary hyperparathyroidism and bone disease. Potassium restriction (limiting bananas, oranges, potatoes) is needed when GFR falls below 30 and potassium excretion is compromised, as hyperkalemia (elevated blood potassium) causes potentially fatal cardiac arrhythmias.