Time of incubation of agar-plate culture for the diagnosis of Strongyloides stercoralis infection.

PURPOSE: Agar-plate culture (APC) remains the most sensitive parasitological technique for S. stercoralis diagnosis. Although it was first described three decades ago, the time of incubation of the plates is neither a commonly described feature nor usually standardized. The aim of the study was to analyze the required time to detect S. stercoralis larvae in APC.

METHODS

A prospective laboratory-based study including all patients with at least one positive APC was performed. The plates were incubated at room temperature for 7 days. Clinical, analytical and parasitological features including results of the direct visualization of the stool (DV) after formalin-ether concentration and time-to-detection (TTD) of the larvae in APC were recorded.

RESULTS

A total of 141 samples from 75 patients had a positive APC. In 49 of them (65.3%) three or more stool samples were processed for direct visualization (DV) and APC. Of these 49 patients, 8 (16.3%) were also diagnosed with DV and 41 (83.7%) were diagnosed only with APC. In 38 samples from 23 (30.7%) patients, the TTD was below 2 days, while in 27 samples from 13 (17.3%) patients, the larvae were detected on the 6th and 7th day.

CONCLUSION

Direct visualization failed to detect S. stercoralis in most of the patients that were diagnosed with APC. Incubation periods below 2 and 5 days would miss an important percentage of infections. At least 7 days of incubation of the APC are required to detect presumably low-burden chronic infections in non-endemic countries.