CPT code 86666 denotes a laboratory immunoassay that evaluates a patient’s serum for antibodies to Ehrlichia, a bacterial genus associated with tick-borne infections. Nationally, serologic testing for tick-borne pathogens is a common component of diagnostic workflows for febrile and exposure-related illnesses; accurate coding for these assays supports clinical decision-making and laboratory reporting. Key payers covered in this analysis include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise overview of the clinical purpose of the assay, typical sites of service, and payer coverage considerations. The publication summarizes reimbursement and billing benchmarks where available, highlights policy and coverage updates affecting laboratory immunoassays, and situates 86666 within clinical testing workflows for suspected Ehrlichia infection. Practical details include typical service contexts, common billing modifiers (listed separately), and guidance on documentation elements typically associated with serologic testing. Data not available in the input is clearly identified in relevant sections.