Can Menstruation Cause False Positive Leukocytes on Urinalysis?
Yes, menstruation is a recognized cause of false-positive leukocyte esterase results on urinalysis and should prompt repeat testing after menses to avoid misdiagnosis of urinary tract infection. 1
Why Menstruation Causes False Positives
Menstrual blood contamination introduces both red blood cells and white blood cells from vaginal/cervical sources into the urine specimen, triggering positive leukocyte esterase reactions that do not represent true pyuria from the urinary tract. 1
The American College of Physicians explicitly identifies menstruation as a benign cause that should prompt repeat evaluation after the cause is excluded, rather than proceeding directly to urologic referral or treatment. 1
Even with "ideal" midstream clean-catch technique in healthy women without UTI, leukocyte esterase was abnormal (>trace) in 35% of specimens, demonstrating the high false-positive rate inherent to this test. 2
Clinical Implications and Management Algorithm
When urinalysis shows positive leukocytes during menstruation:
Do not diagnose or treat UTI based solely on positive leukocyte esterase during menses—repeat the urinalysis 3-7 days after menstruation ends to obtain accurate results. 1
If the patient has acute UTI symptoms (dysuria, frequency, urgency, fever >38.3°C, or gross hematuria), obtain a properly collected specimen via catheterization to bypass vaginal contamination, then proceed with urine culture before starting antibiotics. 3
The combination of positive leukocyte esterase AND positive nitrite increases specificity to 96%, but menstrual contamination can still cause false-positive leukocyte esterase even when nitrite is negative. 3
Evidence on False-Positive Rates
In disease-free women using ideal collection technique, 35% had abnormal leukocyte esterase (>trace) and 27.5% had elevated white blood cells (>5/HPF), demonstrating substantial false-positive rates even without menstruation. 2
The sensitivity of leukocyte esterase ranges from 67-94% with specificity of only 64-92%, meaning approximately 8-36% of positive results are false positives in the general population—this rate increases substantially during menstruation. 1, 3
Critical Pitfalls to Avoid
Never treat based on urinalysis alone during menstruation—the presence of leukocytes requires both clinical symptoms AND confirmation with properly collected specimen after menses or via catheterization. 1, 3
High epithelial cell counts (>few per HPF) indicate vaginal contamination and should trigger repeat specimen collection, as contaminated specimens have false-positive leukocyte esterase rates of 50-65%. 3, 2
The absence of pyuria (negative leukocyte esterase) has excellent negative predictive value (82-91%) for ruling out UTI, but positive results during menstruation lack diagnostic validity without repeat testing. 3
When to Proceed Despite Menstruation
If the patient appears systemically ill with fever, rigors, hemodynamic instability, or suspected pyelonephritis/urosepsis, obtain a catheterized specimen immediately and start empiric antibiotics after culture collection—do not delay treatment waiting for menses to end. 3