Introduction:
Sterile pyuria in the elderly is commonly seen in the Microbiology laboratory. We sought to evaluate a method for excluding fastidious uropathogens, such as Actinotignum schaalii, as a cause for sterile pyuria in this group.
Methods:
As per laboratory protocol, urines are cultured onto horse-blood agar (HBA) and ‘Brilliance UTI’ chromogenic agar then incubated overnight (≥16 hours) in room air. During early March 2017, when negative growth was associated with a urine white cell count ≥50x106/L in a patient ≥65 years old, plates were re-incubated for a further 72 hours in 5% CO2. Additionally, specimens were inoculated onto HBA and chocolate agar (CA) and incubated in 5% CO2 for 72 hours.
Results:
16707 urine samples were processed by the laboratory with 208 (1%) cases of sterile pyuria in the elderly identified. On further incubation, 116 patients (56%) grew a mixed growth of ≥3 distinct organisms with 27 (23%) only evident on the third day. Female gender correlated with higher likelihood of mixed growth (OR 4.67, p=<0.001) and higher epithelial cell count (mean 24.3x106/L vs 3.1x106/L, p=<0.001). A single organism was grown in 24 patients (12%) with 22 of these evident at 48 hours. Of these pure isolates, 9 (4%) were possible uropathogens (with no isolates of Actinotignum schaalii) and 15 were considered contaminants. Incubation in 5% CO2 did not increase yield, with only 3 (non-fastidious) isolates growing only on the recultured HBA/CA plates. True sterile pyuria was seen in 68 patients (33%) with no growth at 72/96 hours, with preponderance in males (OR 3.64, p=<0.001).
Conclusion:
The majority of cases of sterile pyuria at 24 hours did not demonstrate significant growth of a pure pathogen with only 4% growing a possible uropathogen after prolonged incubation. Prolonged incubation of negative plates for a further 24 hours in air may be indicated in cases of sterile pyuria in the elderly but has a low yield in our setting.