Urine Nephrology Now: A Primer for Students in Nephrology
A properly performed and interpreted urinalysis is one of the most powerful and cost-effective tools in nephrology. It provides a real-time "liquid biopsy" of the urinary tract, offering clues to diagnosis, localization of disease, and disease activity.
Color: Normal is pale yellow to amber. Red/brown suggests hematuria, myoglobinuria, or certain drugs. "Cola-colored" urine is classic for acute glomerulonephritis.
Clarity: Normal is clear. Cloudy urine can indicate infection (pyuria), crystals, or amorphous phosphates in alkaline urine.
Specific Gravity: Ranges from 1.003-1.030. A high value (>1.020) indicates concentrated urine (suggesting a prerenal state), while a low, fixed value (~1.010) suggests the kidneys have lost their concentrating ability (seen in advanced CKD or ATN).
| Parameter | Normal | Clinical Significance of a Positive Result |
|---|---|---|
| Protein | Negative | Primarily detects albumin. A key marker for glomerular disease. A 1+ corresponds to ~30 mg/dL. |
| Blood | Negative | Detects heme (from RBCs, hemoglobin, or myoglobin). Requires microscopy to confirm hematuria. |
| Leukocyte Esterase | Negative | Suggests pyuria (WBCs in urine), a marker of inflammation or infection. |
| Nitrites | Negative | Highly specific for infection with nitrate-reducing bacteria (e.g., E. coli). |
| Glucose | Negative | Suggests hyperglycemia exceeding the renal threshold or proximal tubule dysfunction (Fanconi syndrome). |
This is the most crucial part of the urinalysis for a nephrologist. Identifying specific cells and casts can pinpoint the location and type of kidney injury.
Casts are cylindrical structures formed in the renal tubules. Their presence confirms that the elements trapped within them (like RBCs or WBCs) originated from the kidney itself, not the lower urinary tract.
| Finding | Appearance | Pathognomonic Significance |
|---|---|---|
| Dysmorphic RBCs / Acanthocytes | Distorted, misshapen red blood cells. | Bleeding of glomerular origin. |
| Red Blood Cell (RBC) Casts | Cylinders packed with RBCs. | Glomerulonephritis. This is a critical finding. |
| White Blood Cell (WBC) Casts | Cylinders packed with WBCs. | Pyelonephritis or Acute Interstitial Nephritis (AIN). |
| Renal Tubular Epithelial (RTE) Cell Casts | Cylinders containing sloughed tubular cells. | Acute Tubular Necrosis (ATN). "Muddy brown" granular casts are classic for ATN. |
| Granular Casts | Coarse or fine granular cylinders. | |
| Waxy / Broad Casts | Homogenous, sharp-edged, wide cylinders. | Advanced Chronic Kidney Disease. Suggests stasis in dilated tubules. |
| Fatty Casts / Oval Fat Bodies | Casts or cells with refractile lipid droplets ("Maltese cross" under polarized light). | Nephrotic Syndrome. |