Problems of basing patient recruitment for primary care studies on routine laboratory submissions

Use of routine urine submission rates for estimation of patient enrollment in primary care studies of acute urinary symptoms may overestimate patient recruitment rate.

To compare the rates of submission of urines and significant bacteriuria from patients presenting with acute urinary symptoms in study general practices to routine microbiology laboratory urines.

Routine laboratory urine submissions were determined by counting all mid-stream urine specimens submitted to the laboratory from 12 large general practitioner (GP) practices served by Gloucester and Southmead microbiology departments over two years (2000-02). Urine specimens were requested from all patients with acute urinary symptoms referred at research nurse practice visits over the same time period. The annual study urine submission was calculated using the ratio of the number of nurse practice visits to the annual number of possible consulting sessions. Significant bacteriuria was defined as a urine growing a single organism reported as >10(5) colony forming units/ml. Rates per 1000 patients were calculated using practice population data.

The urine submission rate from study patients with acute urinary symptoms was one-third the routine urine submission rate from the same practices. The significant bacteriuria rate attained from the study was less than half the routine significant bacteriuria rate.

Two-thirds of routine urine samples submitted by GPs are probably not for the investigation of acute urinary symptoms. Basing consultation sample size power calculations for primary care studies or sentinel practice-based surveillance in urinary tract infection on routine laboratory submissions is unreliable and will lead to significant overestimation of recruitment rate.