Negative Blood Pad with Many RBCs on Microscopy
A 29-year-old woman at 34 weeks of pregnancy has a routine prenatal urinalysis. The sample is pale yellow, pH 6.0, specific gravity 1.020. Reagent strip: blood negative, protein trace, glucose negative, leukocyte esterase negative, nitrite negative. Microscopy on a 12-mL centrifuged sample: 25–30 RBC/hpf, 0–2 WBC/hpf, no casts, no crystals. She reports no dysuria, flank pain, or known kidney disease, but she takes 2000 mg of vitamin C daily. A repeat fresh sample later that day shows blood 1+ and 5–10 RBC/hpf. Blood pressure is 118/72 mm Hg, and she has no edema.
1. How do you explain the negative blood pad with many RBCs on microscopy?
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The reagent strip blood pad detects heme peroxidase activity from intact RBCs, free hemoglobin, and myoglobin. A false-negative blood pad can occur when a strong reducing agent is present; high-dose ascorbic acid is a classic interferent and can also cause false-negative glucose and false-negative leukocyte esterase results.
Other considerations:
- The pad lyses RBCs, so intact cells should react; a negative pad with visible RBCs is discordant and needs explanation.
- Very dilute or alkaline urine can lyse cells before testing, but this sample has SG 1.020 and pH 6.0, so those are unlikely.
- The repeat sample showing 1+ blood and 5–10 RBC/hpf supports an intermittent or concentration-dependent effect rather than a true absence of blood.
The most likely explanation is ascorbic acid interference with the blood pad, plus possible RBC settling between the strip and microscopy.
2. How would you confirm the cause and what would you report?
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Confirmation:
- Ask the patient to stop vitamin C for 48–72 hours and repeat the urinalysis; the blood pad should become positive.
- Alternatively, test a fresh sample and perform microscopy on the same well-mixed specimen; consider a sulfosalicylic acid protein test to check other strip results if needed.
- Review all strip pads for a pattern of interference (blood, glucose, and leukocyte esterase may all be affected by ascorbic acid).
Reporting:
- Report the microscopy findings, since they are not subject to the same interference.
- Add a comment: "Negative blood pad may be falsely negative due to ascorbic acid; microscopy shows 25–30 RBC/hpf."
- Do not dismiss the hematuria because the dipstick was negative.
3. What is the clinical significance of 25–30 RBC/hpf in this pregnant patient, and what should happen next?
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Microscopic hematuria in pregnancy is not automatically benign. It warrants:
- Repeat urinalysis with a properly collected clean-catch sample to rule out contamination or vaginal bleeding.
- Blood pressure and urine protein assessment for preeclampsia, although her BP is currently normal and protein is only trace.
- Urine culture if there are any urinary symptoms, since pregnancy lowers the threshold for treating bacteriuria.
- Evaluation for stones, infection, and glomerular disease; RBC casts or dysmorphic RBCs would suggest a glomerular source, but none are seen here.
- If hematuria persists postpartum, refer for urologic evaluation.
The laboratory's job is to flag the discrepancy and ensure the clinician sees the microscopy result rather than a falsely negative dipstick.