Performance Assessment on Buffy Coat Examination for Human Granulocytic Anaplasmosis Diagnosis

Introduction/Objective Anaplasma phagocytophilum is an obligate gram-negative intracellular bacterium that causes human granulocytic anaplasmosis (HGA). The diagnostic tests for HGA include buffy coat examination, culture, PCR, and serology. In our lab, suspected patient will be subjected to buffy coat examination, and the result will be further confirmed with PCR. The aim of the study is to assess the epidemiology of HGA diagnosed at WMC during 06/2014- 08/2019, and as well as the performance accuracy of buffy coat examination as compared to PCR. Methods The results of buffy coat examination and in house HGA PCR from 06/2014 to 08/2019 were collected from SoftLAB and SoftMOL, respectively. Results A total of 1,057 blood specimens were tested by buffy coat examination from 06/2014-08/2019; among them, 658 specimens were also analyzed by HGA PCR assay (Fig. 1). Positivity for buffy coat examination and PCR was 2.6% and 6.1%, respectively. Overall, using PCR as reference diagnostic method, 42 new HGA cases were identified. The number of new HGA cases varied among years from 2 to 15, with year of 2017 having the greatest number of total cases (Fig. 2). Further analyzing new HGA cases by months reveal HGA starts in April and ends in December, and peaks at June and July (Fig. 3). When comparing buffy coat result to PCR, 17 positive buffy coat tests were confirmed positive in PCR test, and 619 negative buffy coat tests were also consistent with negative PCR test result. However, 21 cases which were initially negative in buffy coat examination were positive in PCR tests. On the other hand, 1 positive buffy coat examination case was negative in PCR (table 1). Overall, using PCR as a reference test, the sensitivity and specificity of buffy coat examination are 44.7% and 99.8%, respectively. Conclusion Buffy coat examination has high specificity but low sensitivity in detecting HGAcompared to PCR. Low sensitivity is most likely due to variable presence of intracytoplasmic aggregates of HGA in peripheral blood neutrophils and technically incompetency of testing personnel. Overall, it is a good test for laboratory confirmation of clinically suspected HGA cases. We suggest clinicians to add HGA PCR to all the buffy coat examination to avoid missing potential HGA diagnosis. Moreover, more training and competency on testing personnel may improve the sensitivity of buffy coat exam for diagnosis of HGA.