Urine microscopy: cells, casts and crystals

What it is

Urine microscopy is the step that excludes hemoglobinuria and myoglobinuria, because it shows whether there are red cells in the urine at all. Hematuria is more than 2-3 RBCs/HPF.

Casts are cylindrical structures of intratubular precipitation of Tamm-Horsfall mucoprotein. Based on physical appearance they are categorized into cellular, granular, which is either coarse or fine, or waxy. The types of casts, by appearance and by composition and content, have diagnostic value.

How it is diagnosed

The red cells themselves say where the bleeding is:

• Dysmorphic RBCs and RBC casts point to glomerular bleeding
• Isomorphic RBCs with no casts point to extraglomerular bleeding

The presence of eosinophils indicates AIN or athero-embolic disease.

The casts are read as follows:

• Hyaline casts: physiologic, seen with concentrated urine, fever and exercise
• RBC casts: glomerular bleeding, so GN or vasculitis
• WBC casts: infection, meaning pyelonephritis, or inflammation, meaning interstitial nephritis
• Pigmented granular casts, also called heme granular casts or muddy brown casts: ATN or acute GN
• Fatty casts: nephrotic syndrome, above 3.5 g/d

On a microscopy panel the things to name are squamous epithelial cells, transitional epithelial cells, renal tubular epithelial cells (RTEC), monomorphic and dysmorphic RBCs, WBC, and then the casts themselves: hyaline, granular, WBC, RBC, fatty, waxy and RTEC casts.