Degree and frequency of inhibition in a routine real-time PCR detecting Pneumocystis jirovecii for the diagnosis of Pneumocystis pneumonia in Turkey.

Routine laboratory diagnosis of Pneumocystis jirovecii is currently achieved by PCR in almost all laboratories with sufficient equipment due to its high sensitivity and specificity compared to staining methods. A current issue that limits the reliability and sensitivity of PCR is the degree of inhibition caused by inhibitory substances in respiratory samples. The present study aimed to analyse the degree and frequency of inhibition in real-time PCR detecting P. jirovecii in respiratory specimens submitted to a Pneumocystis pneumonia (PcP) diagnosis laboratory in Ege University Medical School, Turkey. Between July 2009 and December 2010, 76 respiratory specimens [63 bronchoalveolar lavage (BAL) fluid, 10 sputum samples, two tracheal aspiration fluid and one thoracentesis fluid] obtained from 69 PcP-suspected patients were investigated for the presence of P. jirovecii using real-time PCR targeting the cdc2 gene. Of these samples, 42 of the specimens were stained and examined by microscopy according to the request of the clinicians. PCR was positive in 15 specimens in the initial run. Of the remaining 61 samples, 41 of them were negative with positive internal inhibition controls (i.e. true-negative group). The frequency of inhibition in the initial run was 26.31 % (20/76) as determined by spiked negative controls. All of the inhibited samples were resolved after 1 : 2, 1 : 5, 1 : 10 and 1 : 20 dilutions. P. jirovecii was detected by PCR in two inhibited specimens after retesting with diluted samples which were also positive by microscopy. The incidence of P. jirovecii in respiratory specimens was 22.36 % (17/76) as determined by real-time PCR and 7.14 % (3/42) by microscopy. Overall, the incidence of P. jirovecii in respiratory samples was 23.68 % (18/76) as detected by both methods. In conclusion, inclusion of spiked positive controls in each sample and retesting with diluted samples to resolve inhibition increased the reliability of the real-time PCR assay in terms of determining false-negative results and influencing the treatment of the patient. Furthermore, results of the present study determined for the first time the frequency and degree of inhibition in a real-time PCR detecting P. jirovecii in respiratory specimens during routine diagnosis of PcP.