Diagnostic Testing Characteristics of Routine Laboratory Findings for Suspected Human Granulocytic Anaplasmosis

Background: Previous studies have identified routine laboratory abnormalities associated with human granulocytic anaplasmosis (HGA). To aid clinicians in evaluating patients with suspected disease while waiting for polymerase chain reaction (PCR) results, we describe the diagnostic testing characteristics of routine laboratory tests compared with a gold standard PCR. Design: We conducted a retrospective chart review of emergency department patients in a highly endemic region who had an HGA PCR sent as part of their clinical care. We reviewed 223 eligible PCR-positive cases and a randomly sampled selection of 466 eligible PCR-negative cases. We calculated the positive and negative likelihood ratios (LR+ and LR−) for common laboratory abnormalities (leukopenia, thrombocytopenia, hyponatremia, bandemia, or transaminitis) alone and in combination with each other. Results: The LR+ and LR− were statistically significant for each laboratory test individually and in combination with each other. Individual laboratory test LRs were: leukopenia, LR+ = 2.9, LR− = 0.74; thrombocytopenia, LR+ = 3.51, LR− = 0.41; bandemia, LR+ = 5.16, LR− = 0.69; hyponatremia, LR+ = 2.37, LR− = 0.46; and transaminitis, LR+ = 2.13, LR− = 0.64. Laboratory test combination LR+s were at least 2 laboratory abnormalities LR+ = 2.93, at least 3 laboratory abnormalities LR+ = 6.38, at least 4 laboratory abnormalities LR+ = 12.96, and all 5 laboratory abnormalities LR+ = 39.7. When no laboratory tests were positive, the LR− was 0.16. Conclusion: Laboratory abnormalities (leukopenia, thrombocytopenia, bandemia, hyponatremia, and transaminitis) alone were helpful in ruling in disease in patients with suspected disease, with greater than four or more laboratory values abnormal strongly suggestive for HGA. All five laboratory tests being normal strongly helped rule out disease.