WHAT'S KNOWN ON THE SUBJECT? AND WHAT DOES THE STUDY ADD?: Microscopic pyuria is widely used as a surrogate marker of infection, although there is little data supporting its use in patients who present with non-acute LUTS. The effects of urinary storage, preservation, and the use of laboratory methods to enhance leucocyte detection, are also unclear. This large, prospective study highlights the poor performance of dipstick urine analysis, and direct microscopy, as surrogate markers of UTI in patients with LUTS. A series of laboratory analyses also examine the effects of urine handling and processing on test integrity, which have important implications for clinical practice.
Objective: To evaluate the diagnostic performance of pyuria as a surrogate marker of urinary tract infection (UTI) in patients with chronic lower urinary tract symptoms (LUTS), and determine the impact of sample storage, cytocentrifugation, and staining techniques, on test performance.
Patients and methods: Between 2008 and 2011, we recruited 1223 patients (120 men; 1103 women; mean age 54 years) with one or more LUTS from a specialist urological outpatient service. We conducted a prospective observational study to determine the performance of microscopic pyuria ≥10 wbc/μL as a surrogate marker of UTI in patients with LUTS. All patients provided clean-catch midstream urine (MSU) samples for analysis, and routine microbiological cultures were used as our reference standard. We also scrutinised the performance of dipstick leucocyte esterase ≥ 'trace' in the detection of microscopic pyuria. The influence of sample handling and processing on test performance was examined in a series of laboratory studies. The effects of storage on leucocyte decay were determined using repeated microscopic assessments of individual urine samples, to plot temporal changes in leucocyte numbers. This study used varied storage conditions (≈20 °C and 4 °C), and boric acid preservation. Paired microscopic assessments were used to determine the effects of centrifugation on leucocyte salvage in spun/unspun samples (relative centrifugal force range 39-157 g). Similar methods were used to assess microscopic leucocyte quantification in stained/unstained urine (Sternheimer-Malbin protocol).
Results: The positive predictive value (PPV) and negative predictive value (NPV) of pyuria as a surrogate marker of UTI were 0.40 (95% confidence interval [CI] 0.37-0.43) and 0.75 (95% CI 0.73-0.76), respectively. The dipstick was unable to identify significant microscopic pyuria (≥10 wbc/μL) in 60% of the samples: PPV 0.51 (95% CI 0.48-0.55); NPV 0.75 (95% CI 0.73-0.76). Microscopic pyuria performed poorly as a surrogate of UTI defined by bacterial culture. Whilst refrigeration and preservation did retard leucocyte loss (F = 11; DF = 2; P < 0.001), 40% of cells were still lost by 4 h. Centrifugation had an unpredictable influence on cell salvage (coefficient of variation 5750%) and the use of staining to improve leucocyte detection proved ineffective (Z = -0.356; P = 0.72).
Conclusions: Pyuria performs badly as a surrogate of UTI in patients with LUTS. This is exacerbated by cell loss during storage, and neither centrifugation, nor staining, appears to confer any diagnostic advantage. Clinicians should be alerted to the significant limitations of these tests.
© 2013 BJU International.