Harmonization of PCR-based detection of intestinal pathogens: experiences from the Dutch external quality assessment scheme on molecular diagnosis of protozoa in stool samples

Background: Real-time PCR methods are increasingly used in routine patient care settings not only to determine the presence or absence of pathogens in patient materials, but also to obtain semiquantitative results to estimate the pathogen load. However, it is so far unknown how well these methods are harmonized among different laboratories. Methods: Sets of stool samples were distributed three to four times per year to ca. 25–40 participating laboratories within the European Union as part of an external quality assessment scheme (EQAS) for the detection of gastrointestinal protozoa. This paper presents the results obtained over a 3-year period for Entamoeba histolytica, Entamoeba dispar, Giardia lamblia, Cryptosporidium species and Dientamoeba fragilis. Results: Although both false-positive and false-negative results were reported, the overall sensitivity and specificity were high. The substantial differences in the quantitative output of the real-time PCR assays could be traced back to differences in DNA isolation procedures between different laboratories. Conclusions: Participation in an EQAS proved to be important as it provides information on how the real-time PCR methods used by the participant compares to the generally reported results and indicates how procedures could be improved. Semiquantitative results of real-time PCR methods are not exchangeable between laboratories as long as the diagnostic procedures are not harmonized. Intralaboratory comparison of semiquantitative real-time PCR results seems only possible by the use of calibration curves derived from well-validated standards in clinical material and not by spiking solutions with purified DNA.