ASCP Clinical Microscopy 1 — Questions and Answers

Question 1: In a urinalysis, what does a positive reagent strip (dipstick) result for leukocyte esterase indicate?

Intact red blood cells in urine
The presence of white blood cells (neutrophils) or their breakdown products (Correct answer)
Glucose in the urine
Bacterial protein in the urine

Correct answer: The presence of white blood cells (neutrophils) or their breakdown products

Leukocyte esterase is an enzyme found in neutrophil granules. A positive dipstick result indicates WBCs (pyuria) or lysed neutrophils. It is a screening test for urinary tract infection but requires microscopic confirmation.

Leukocyte esterase dipstick: esterase released from neutrophils (intact or lysed) cleaves an ester substrate causing a color change to purple. Sensitivity for UTI is approximately 80–90% and specificity is approximately 90%. False negatives occur with very dilute urine, high glucose, oxalic acid, and certain antibiotics at high concentrations including cephalexin, tetracycline, and gentamicin. False positives occur from formalin contamination or vaginal contamination with WBCs. Significant pyuria is defined as 5 or more WBCs per HPF on microscopy. Leukocyte esterase positive plus nitrite positive has high predictive value for UTI. Sterile pyuria (WBCs without bacteria) is seen in TB, interstitial nephritis, and urolithiasis.

Question 2: Which urinary casts are characteristic of acute tubular necrosis (ATN)?

Hyaline casts
Granular (muddy brown) casts (Correct answer)
Red blood cell casts
Waxy casts

Correct answer: Granular (muddy brown) casts

Granular muddy brown casts in urine are characteristic of acute tubular necrosis. They form from degenerating tubular epithelial cells and cellular debris in the renal tubular lumen, creating the coarsely granular appearance.

Urinary cast formation: Tamm-Horsfall protein (uromodulin) secreted by the thick ascending limb of Henle forms the matrix. Cast types and clinical correlations: Hyaline casts come from concentrated urine, exercise, or fever and are benign. RBC casts are seen in glomerulonephritis and vasculitis and are pathognomonic of glomerular bleeding. WBC casts indicate pyelonephritis or interstitial nephritis. Granular (muddy brown) casts are seen in ATN, ischemic or nephrotoxic injury from aminoglycosides, contrast, or myoglobin. Waxy casts are seen in advanced CKD and renal failure where wide casts indicate dilated atrophied tubules. Fatty casts are seen in nephrotic syndrome with Maltese cross polarization.

Question 3: Which stain is used to identify fat/lipid in urine sediment?

Gram stain
Oil Red O or Sudan III stain (Correct answer)
Wright-Giemsa stain
Prussian blue

Correct answer: Oil Red O or Sudan III stain

Oil Red O (and Sudan III/IV) are lipophilic dyes that stain neutral fats (triglycerides and cholesterol esters) orange-red. They are used to confirm lipiduria (oval fat bodies, fatty casts) in nephrotic syndrome or tubular injury.

Lipiduria evaluation uses two complementary techniques. Polarized light microscopy: cholesterol esters in oval fat bodies create birefringent Maltese cross pattern under polarized light (pathognomonic). Oil Red O stains neutral lipids (triglycerides and cholesterol esters) in oval fat bodies, fatty casts, and free lipid droplets. Clinical context: nephrotic syndrome with heavy proteinuria causes lipiduria and oval fat bodies, seen in lupus nephritis, minimal change disease, and membranous nephropathy. Mechanism: hypoalbuminemia leads to hepatic overproduction of lipoproteins, lipoproteinuria, renal tubular reabsorption of lipids, shedding of tubular cells as oval fat bodies, and lipid cast formation.

Question 4: A WBC cast in urine sediment is most indicative of:

Acute glomerulonephritis
Pyelonephritis or acute interstitial nephritis (Correct answer)
Nephrotic syndrome
Prerenal azotemia

Correct answer: Pyelonephritis or acute interstitial nephritis

WBC (leukocyte) casts indicate that inflammation is occurring within the renal tubules and parenchyma, characteristic of pyelonephritis (bacterial) or acute interstitial nephritis (drug-induced or autoimmune). They must be distinguished from clumped WBCs not in cast matrix.

WBC cast identification: must see WBCs embedded in cast matrix (Tamm-Horsfall protein). Staining with Wright-Giemsa or Hansel stain differentiates neutrophils (PMNs with multilobed nuclei) from eosinophils (bilobed with orange granules). PMN casts: bacterial pyelonephritis, lupus nephritis, renal transplant rejection. Eosinophil casts (rare): allergic interstitial nephritis from NSAIDs, penicillins, cephalosporins, or PPIs. Eosinophiluria on Hansel stain may also be seen in atheroembolic renal disease. WBC cast versus clump: cast has sharply defined parallel edges with embedded cells; clump is amorphous.

Question 5: Spermatozoa seen on a urine microscopy report in a female patient most likely indicates:

Urinary tract infection by unusual organisms
Specimen contamination from recent sexual intercourse or vesico-vaginal communication (Correct answer)
Renal tubular injury
Glomerular disease

Correct answer: Specimen contamination from recent sexual intercourse or vesico-vaginal communication

Spermatozoa in female urine almost always represent contamination from recent sexual intercourse (retrograde into bladder, vaginal contamination of midstream specimen, or very rarely from vesico-vaginal fistula). They have no diagnostic significance for urinary tract disease.

Spermatozoa in urine microscopy in females indicates contamination from vaginal reflux, midstream clean-catch contamination during or after intercourse, or rarely atrioventricular fistula. In males: retrograde ejaculation (into bladder), recent ejaculation (sperm can persist 24–48 hours in urine), or anejaculation. Spermatozoa are easily recognized: oval head approximately 5 by 3 µm with characteristic pointed acrosomal cap, midpiece, and long flagellar tail. Reporting: note as incidental finding and indicate likely contamination in females.

Question 6: A patient's CSF appears xanthochromic (yellow). What does this finding most commonly indicate?

Traumatic tap during lumbar puncture
Subarachnoid hemorrhage (SAH) that occurred hours to days before the procedure (Correct answer)
Normal variant in elderly patients
Bacterial meningitis

Correct answer: Subarachnoid hemorrhage (SAH) that occurred hours to days before the procedure

Xanthochromia (yellow discoloration of CSF) results from hemoglobin breakdown products (oxyhemoglobin within 2 hours; bilirubin by 10–12 hours) after subarachnoid hemorrhage. It peaks at 48 hours and persists for 2–4 weeks — distinguishing SAH from traumatic tap.

CSF xanthochromia differentiation: Traumatic tap: bloody in first tube, clears progressively in subsequent tubes; no xanthochromia (blood just introduced, no time for hemoglobin catabolism); RBCs clot. SAH: uniformly bloody in all tubes (blood circulates throughout CSF space); xanthochromic supernatant after centrifugation; no clotting (CSF RBCs lyse). Timing of xanthochromia: oxyhemoglobin (pink) appears 2–4 hours post-SAH; bilirubin (yellow) appears 10–12 hours post-SAH from RBC catabolism by macrophages; peaks at 48 hours; persists 2–4 weeks. Spectrophotometry is more sensitive than visual inspection. Other causes of xanthochromia: hyperbilirubinemia, elevated CSF protein above 150 mg/dL, and spinal block.

ASCP Clinical Microscopy 1

In a urinalysis, what does a positive reagent strip (dipstick) result for leukocyte esterase indicate?