Anaplasma phagocytophilum is a bacterium carried by blacklegged ticks (also called deer ticks), the same species that can transmit Lyme disease. When this organism infects humans, it causes a condition called human granulocytic anaplasmosis (HGA), which often begins with fever, chills, muscle aches, and fatigue. Left untreated, it can lead to more serious complications such as low white blood cell counts, liver inflammation, or, in rare cases, severe multi-organ illness.
This panel measures two types of antibodies: IgM and IgG. Antibodies are proteins made by the immune system to recognize and fight infections. IgM antibodies are usually the first to appear, often within the first one to two weeks of infection, and their presence suggests a recent or active infection. IgG antibodies take longer to develop, typically appearing a few weeks after infection, and may remain detectable for months or even years. A positive IgG result therefore often indicates a past exposure or, in some cases, an ongoing infection.
Interpreting this test requires some nuance. Because IgM can sometimes produce false positives, reacting even when infection is not truly present, doctors usually confirm results with additional testing, such as repeat antibody testing over time (to check for rising IgG levels) or direct detection of the bacteria by polymerase chain reaction (PCR). Similarly, a single positive IgG result may simply mean prior exposure, not necessarily current illness, so context from symptoms and other lab findings is essential.
This panel is especially useful for people who develop fever and flu-like illness after a tick bite, or for those living in or traveling to areas where Anaplasma is common. When paired with PCR or repeat testing, it helps clarify the timing and likelihood of infection, guiding whether antibiotic treatment is needed.
