White Blood Cells in Urine During Menstruation
White blood cells in urine are NOT a normal finding in menstruating women and should prompt evaluation for urinary tract infection or other pathology, though menstrual contamination can cause false-positive results.
Understanding Normal WBC Thresholds in Urine
The presence of WBCs in urine (pyuria) indicates inflammation or infection of the urinary tract, regardless of menstrual status:
- Uninfected urine typically contains ≤10 WBCs/mm³ (or ≤1,000 WBCs/mL) by hemocytometer 1
- Manual microscopy shows ≥8 WBCs per high-power field as significant for bacteriuria 2
- Automated microscopy identifies >2 WBCs/hpf as significant pyuria indicative of urinary tract inflammation 2
- Infected urine regularly contains >10,000 WBCs/mL, with mean counts of 310,000 WBCs/mL in infected patients 1
The Menstrual Contamination Problem
While menstruation itself does not cause true pyuria, menstrual blood can contaminate urine specimens and create misleading results:
- Menstrual discharge contains only 36% blood on average (range 1.6-81.7%), with the remainder being endometrial tissue fluid 3
- Blood contains white blood cells that can appear in contaminated urine samples, creating false-positive pyuria 3
- Elevated epithelial cell counts in urine are significantly associated with contaminated samples 4
Clinical Approach to WBCs in Urine During Menstruation
Step 1: Assess for Contamination
- High epithelial cell counts suggest specimen contamination rather than true infection 4
- If contamination is suspected, obtain a clean-catch midstream specimen or consider catheterization for accurate results 2
Step 2: Evaluate for Urinary Tract Infection
- Pyuria (≥8-10 WBCs/hpf) is the best determinant of bacteriuria requiring therapy 2
- In uncomplicated symptomatic UTI, positive nitrites and leukocyte esterase on dipstick can be treated without culture 2
- The absence of pyuria provides strong evidence against urinary tract infection 1
Step 3: Consider Alternative Diagnoses if Symptomatic
If the patient has pelvic pain or abnormal vaginal discharge along with WBCs:
- Evaluate for pelvic inflammatory disease (PID) if there is uterine/adnexal tenderness or cervical motion tenderness 5, 6
- WBCs on saline microscopy of vaginal secretions (not urine) support PID diagnosis 5, 6
- Test for N. gonorrhoeae and C. trachomatis with nucleic acid amplification testing 5, 7
Critical Pitfall to Avoid
Do not dismiss pyuria as "normal menstrual contamination" without proper evaluation. True pyuria indicates pathology and requires investigation for UTI or other urinary tract inflammation, even during menstruation 2, 1. If initial results are equivocal due to suspected contamination, repeat the urinalysis with a properly collected specimen rather than assuming the WBCs are menstrual in origin.