Comparison of a quantitative PCR assay with peripheral blood smear examination for detection and quantitation of Babesia microti infection in humans

• A real-time quantitative PCR was developed to detect the 18S rRNA gene of Babesia microti.
• The DNA copy number correlated with the number of infected erythrocytes seen on blood smear (P < 0.001).
• qPCR was able to detect parasitic DNA in patients who had become negative on smear examination.
• qPCR may permit systematic evaluations of the relative efficacy of anti-Babesia drug regimens.
• DNA detection per se, however, does not establish the presence of viable parasites.

Using a real-time quantitative PCR (qPCR), we determined the number of DNA copies/mL of blood of a Babesia microti gene in infected patients. Thirty-six patients (whose median age was 62.5 years and 75.0% were male) with at least 1 qPCR-positive blood sample were included in this analysis, including 16 with serial blood samples. Based on testing of serial blood samples, it could be demonstrated that the smear became negative while the qPCR remained positive. A moderate to strong correlation was found between the DNA copy number and the number of infected erythrocytes per milliliter of blood (Pearson's r = 0.68, P < 0.001). Based on limited data, the DNA copy number fell by a mean of 4.1–12.9% per day on active treatment and by 3.5–7.1% per day off therapy. qPCR methodology may permit systematic evaluations of the relative efficacy of various antiparasitic drug regimens and other therapeutic modalities, although a limitation of such testing is that DNA detection per se does not establish the presence of viable parasites.