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Urine microscopy
What are the important aspects of urine microscopy?
Urinary Casts
Urinary casts are cylindrical structures that are formed from coagulated protein (Tamm-Horsfall protein) secreted by tubular cells.
As their name implies, they are usually formed in the long, thin, hollow renal tubules and take their shape. They develop in the distal convoluted tubule or the collecting duct.
The proximal convoluted tubule and loop of Henle are not locations for cast formation.
Low urine flow rate, high urinary salt concentration, and low urine pH all favor protein denaturation and precipitation of Tamm-Horsfall protein, the organic matrix that cements the casts together
Urinary Casts
| URINARY CASTS | DISEASE ASSOCIATIONS |
|---|---|
| Hyaline | May be nonspecific; seen in normal individuals or during severe intravascular volume depletion (after strenuous exercise or with diuretic use), first morning specimens, acidic and concentrated urine |
| Granular | May be nonspecific, result from the breakdown of cellular casts “Muddy-brown” heme-granular casts are seen in acute tubular necrosis |
| Waxy and broad | Advanced kidney disease |
| Red blood cell | Glomerulonephritis |
| White blood cell | Urinary tract infections (pyelonephritis, cystitis), tubulointerstitial nephritis, renal tuberculosis |
| Fatty | Nephrotic syndrome (“Maltese cross” appearance under polarized light) |
Cells in Urine
The presence of more than 15 to 20 squamous epithelial cells/low power field (lpf) is an indication that the urine specimen is contaminated.
The presence of dysmorphic RBCs (also called acanthocytes and/or RBC casts is the sine qua non of glomerulonephritis. The presence of no more than two to five WBCs/hpf is considered normal.
WBCs in the urine (pyuria) may be indicative of a UTI. If the urine culture is negative (i.e., sterile pyuria), this could indicate either an infection that is not easily cultured or noninfectious causes such as urinary tract stones, interstitial nephritis, or acute glomerulonephritis. However, when associated with other cellular elements or debris, pyuria may have limited diagnostic value.
Crystals in Urine
| URINARY CRYSTALS | DESCRIPTION | DISEASE ASSOCIATION |
|---|---|---|
| Calcium oxalate dihydrate | Envelope shaped | Ethylene glycol toxicity |
| Calcium oxalate monohydrate | Dumbbell shaped | Ethylene glycol toxicity |
| Uric acid | Diamond or barrel shaped | Hyperuricosuria |
| Triple phosphate (also called struvite) or magnesium ammonium phosphate | Coffin lid | Urinary tract infection caused by urea-splitting organisms (Proteus, Klebsiella) |
| Cystine | Hexagonal | Cystinuria |
| Indinavir | Flat rectangular plates, fan shaped, or starburst in appearance | Indinavir therapy |
Bacteria in Urine
Because of the abundance of normal microbial flora in the vagina and/or external urethral meatus, it is not uncommon to see bacteria in urine specimens. Similarly, if the urine specimen is left standing at room temperature for a considerable period, bacteria can multiply rapidly (see Fig. 2.2 F). Therefore the identification of various microbial organisms (on Gram staining) in any urine specimen should be interpreted cautiously. (See Chapter 46 for discussion regarding asymptomatic bacteriuria.) The diagnosis of bacteriuria in those with suspected UTI should be followed by a urine culture. In general, the presence of more than 100,000 colony forming units (CFUs)/mL of a single organism reflects significant bacteriuria. The presence of multiple organisms usually reflects polymicrobial contamination. However, the presence of any organism obtained in a specimen via a catheter or suprapubic aspiration should be considered significant.
Yeasts in Urine
The presence of yeast cells usually represents contamination or can indicate a true infection. They are often difficult to distinguish from red cells and amorphous crystals but can be identified by their tendency to bud. Most often they are Candida, which may colonize the bladder, urethra, or vagina.
