Background The cut-off of ≥5 polymorphs/high power field (p/hpf) in a Gram stained male urethral smear (GSUS) for diagnosing non-gonococcal urethritis derives from evaluations using Chlamydia trachomatis (CT) culture as gold standard. There is also significant observer variation in performing microscopy.
Objective To compare automated flow cytometry of first void urine white cell count (FVU-UWC) to GSUS for prediction of CT infection in men.
Methods First void urines from 227 symptomatic male patients, all undergoing GSUS, were collected and patients stratified into: those treated for urethritis, diagnosed by GSUS result, history and examination (n=114) and; those not thought to have urethritis (n=113). First void urines were analysed by automated flow cytometry using the bench-top Sysmex UF-100 Analyser.
Results CT was found in 11.6% and 6.2% urethritis and non-urethritis patients respectively and gonorrhoea (GC) in 3.6% and 1.1% respectively. There was no difference in UWC between urethritis and non-urethritis (p=0.690) nor association between GSUS grade and UWC (p=0.933). Median GSUS was higher for CT positive compared to negative patients (≥5/hpf vs 0). UWC were higher in CT (p=0.001) and GC (p≤0.001) positives. ROC area under the curve (AUC) for predicting CT was 0.844 (p≤0.001) with an optimal cut-off of >29 UWC/μl giving sensitivity of 90% and specificity of 76%. For predicting CT in urethritis and non-urethritis: the positive predictive values using the >29 UWC/μl cut-off were 20% (95%CI 9.1% to 37.5%) and 34.4% (19.2% to 53.2%) and the negative predictive values NPVs 100% (94.2% to 100%) and 97.5% (90.4% to 99.6%) respectively; for GSUS, using ≥5/hpf, positive predictive values were 10.5% (5.4% to 18.9%) and 0% (0% to 60.4%) and negative predictive values were 82.4% (55.8% to 95.3%) and 93.6% (86.8% to 97.2%) respectively.
Conclusion UWC is (1) higher in CT and GC infection; (2) is a better predictor of urethral CT than GSUS and; (3) possibly more useful for determining non-gonococcal urethritis. Automated rapid flow cytometry may offer significantly improved utility over microscopy in the clinic.
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