Urine Nephrology Now: A Primer for Students in Nephrology
A comprehensive assessment of kidney health requires a panel of tests that evaluate filtration, tubular function, and metabolic homeostasis.
Blood Urea Nitrogen (BUN): Measures nitrogen waste from protein metabolism. Elevated levels can indicate decreased kidney function, dehydration, or increased protein catabolism. A BUN/Cr ratio >20:1 suggests a prerenal state.
Serum Creatinine: A waste product of muscle metabolism that provides a reliable marker of kidney function. Rising levels indicate declining GFR, though it is influenced by muscle mass.
eGFR (estimated Glomerular Filtration Rate): An estimate of kidney filtering capacity based on creatinine, age, and sex. It is the primary metric for staging CKD.
Serum Electrolytes (Na, K, Cl, CO2): Reveals the kidney's ability to maintain fluid and electrolyte balance. Abnormalities can suggest kidney dysfunction or specific tubular disorders.
Cystatin C: A protein produced by all nucleated cells, Cystatin C is less affected by muscle mass than creatinine, making it a more accurate GFR marker in certain populations (e.g., elderly, amputees).
Essential for diagnosing and managing Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD).
Serum Calcium: Evaluates parathyroid function and MBD. Must be corrected for albumin.
Serum Phosphorus: Levels typically rise in advanced CKD and contribute to vascular calcification and secondary hyperparathyroidism.
Magnesium (Mg): Assesses for electrolyte imbalances that can occur in kidney disease or with certain medications, affecting neuromuscular and cardiac function.
Intact Parathyroid Hormone (iPTH): Measures the severity of secondary hyperparathyroidism in CKD, guiding treatment decisions.
25-hydroxy Vitamin D: Assesses nutritional status and guides supplementation, as its activation is impaired in kidney disease.
A properly performed and interpreted urinalysis is a "liquid biopsy," offering clues to diagnosis, localization of disease, and disease activity. Key findings are detailed in Chapter 4.
These calculations are crucial in the workup of AKI and electrolyte disorders.
Fractional Excretion of Sodium (FENa) = (Urine Na × Serum Cr) / (Serum Na × Urine Cr) × 100
A FENa <1% suggests a prerenal state. A FENa >2% suggests tubular damage (ATN).
Fractional Excretion of Urea (FEUrea) = (Urine Urea × Serum Cr) / (Serum Urea × Urine Cr) × 100
FEUrea is a useful alternative to FENa in patients on diuretics. An FEUrea <35% suggests a prerenal cause.
These tests are ordered when there is suspicion for glomerular disease, vasculitis, or a paraprotein-related process.
| Test | Associated Disease(s) | Clinical Utility |
|---|---|---|
| Anti-Nuclear Antibody (ANA) | Systemic Lupus Erythematosus (SLE) | Sensitive but not specific screening test for autoimmune diseases. |
| Anti-dsDNA | Lupus Nephritis | Highly specific for SLE and correlates with disease activity. |
| Complement (C3, C4) | Lupus Nephritis, Post-infectious GN, MPGN | Low levels suggest immune complex-mediated disease. |
| ANCA (c-ANCA/PR3, p-ANCA/MPO) | Small Vessel Vasculitis (GPA, MPA) | Key for diagnosing pauci-immune rapidly progressive glomerulonephritis. |
| Anti-GBM Antibody | Goodpasture's Disease | Highly specific for anti-glomerular basement membrane disease. |
| Anti-PLA2R Antibody | Primary Membranous Nephropathy | Highly specific; can diagnose primary MN without a biopsy in many cases. |