Performance evaluation of automated urine microscopy as a rapid, non-invasive approach for the diagnosis of non-gonococcal urethritis

Abstract

Objectives: Gram-stained urethral smear (GSUS), the standard point-of-care test for non-gonococcal urethritis (NGU) is operator dependent and poorly specific. The performance of rapid automated urine flow cytometry (AUFC) of first void urine (FVU) white cell counts (UWCC) for predicting Mycoplasma genitalium and Chlamydia trachomatis urethral infections was assessed and its application to asymptomatic infection was evaluated.

Methods: Receiver operating characteristic curve analysis, determining FVU-UWCC threshold for predicting M. genitalium or C. trachomatis infection was performed on 208 'training' samples from symptomatic patients and subsequently validated using 228 additional FVUs obtained from prospective unselected patients.

Results: An optimal diagnostic threshold of >29 UWC/µL gave sensitivities and specificities for either infection of 81.5% (95% CI 65.1% to 91.6%) and 85.8% (79.5% to 90.4%), respectively, compared with 86.8% (71.1% to 95%) and 64.7% (56.9% to 71.7%), respectively, for GSUS, using the training set samples. FVU-UWCC demonstrated sensitivities and specificities of 69.2% (95% CI 48.1% to 84.9%) and 92% (87.2% to 95.2%), respectively, when using validation samples. In asymptomatic patients where GSUS was not used, AUFC would have enabled more infections to be detected compared with clinical considerations only (71.4% vs 28.6%; p=0.03). The correlation between UWCC and bacterial load was stronger for M. genitalium compared with C. trachomatis (τ=0.426, p≤0.001 vs τ=0.295, p=0.022, respectively).

Conclusions: AUFC offers improved specificity over microscopy for predicting C. trachomatis or M. genitalium infection. Universal AUFC may enable non-invasive diagnosis of asymptomatic NGU at the PoC. The degree of urethral inflammation exhibits a stronger association with pathogen load for M. genitalium compared with C. trachomatis.

Keywords: CHLAMYDIA TRACHOMATIS; DIAGNOSIS; M GENITALIUM; MOLECULAR TECHNIQUES; URETHRITIS.

Figure 1

Box plots comparing the distributions of first void urinary white cell counts (UWCC) per μL in patients with and without urethritis stratified by urethral smear grade (A) and C. trachomatis or M. genitalium infection status (B). Training set patients only. The median urinary white cell count increases as the urethral smear grade increases (A, p≤0.001). UWCC are increased in infected patients with respect to non-infected patients (B, p≤0.001), regardless of clinical grouping. Negative Gram-stained urethral smear (GSUS): ‘−’ (0 PMN/HPF), ‘−/+’ (1–5 PMN/HPF). Positive GSUS: ‘+’ (5–10 PMN/HPF), ‘++’ (10–20 PMN/HPF) and ‘+++’ (>20 PMN/HPF). PMN/HPF, polymorphonuclear cells per high power field.