A. Phagocytophilum IgG Titer

An unexplained fever after tick exposure can leave you wondering what caused it. This test looks for IgG antibodies that react with Anaplasma phagocytophilum, which causes human granulocytic anaplasmosis. It can help confirm that diagnosis when results from the illness and recovery are compared. A single positive result usually can't establish what caused a particular episode.

Timing changes the interpretation. Antibodies may still be absent early in the illness and remain detectable long after recovery. This is an established diagnostic test when used with your symptoms, exposure history, and other findings. Its value as routine screening when you're healthy hasn't been established.

What the Number Means

IgG is a class of antibody that can persist after an infection. This test measures the reaction of those antibodies in serum. Serum is the liquid separated from blood after it clots.

The method is called indirect immunofluorescence, abbreviated IFA. Increasingly diluted serum is placed on slides containing cells infected with the bacterium. A fluorescent label makes bound IgG visible under a microscope. Your titer is the greatest dilution at which the characteristic reaction remains visible.

For example, a result written as 1:128 means the reaction remained detectable after a 128-fold dilution. A larger denominator indicates stronger antibody reactivity within the same assay. It doesn't measure how many bacteria are in your body or how severe the illness is.

A fourfold rise between appropriately timed illness and recovery samples provides confirmatory evidence of recent infection when your illness fits anaplasmosis. A stable result is less informative. It can reflect past exposure, but it can also occur when the first sample was collected after antibodies had already risen.

Human Granulocytic Anaplasmosis

Anaplasmosis is spread mainly by Ixodes ticks. In the United States, these include the blacklegged ticks that also transmit Lyme disease. The bacterium infects neutrophils. These white blood cells help defend you against infection.

The illness commonly causes fever, headache, chills, and muscle aches. Blood work often shows low platelets, low white blood cells, and raised liver enzymes. Those findings after possible tick exposure raise suspicion, though none is specific to anaplasmosis. Many people don't remember a tick bite.

Severe disease can involve kidney failure, breathing failure, or failure of several organs. A systematic review found complications in about 40% of published cases with individual clinical information, and deaths in about 3%. Published cases overrepresent severe and unusual illness, so those figures aren't estimates of your risk after a tick bite. U.S. clinical guidance describes deaths in fewer than 1% of recognized cases.

The same tick can transmit more than one infection, including Lyme disease and babesiosis. How often coinfection occurs varies by location and the population studied. Antibodies against several organisms can also reflect separate past exposures, so multiple positive antibody results don't establish simultaneous infections.

Why an Early Sample Is Often Negative

In a Korean hospital cohort, about 24% of people with confirmed anaplasmosis had detectable IgG at admission. Among those tested four weeks after symptoms began, about 92% were positive. The study was small, and the number of people tested changed during follow-up, but it directly illustrates the delay in the serum IgG response.

A separate study of 38 people with infection confirmed by both PCR and culture found antibodies in about 55% at presentation. That assay measured IgG, IgM, and IgA together, so its sensitivity is not an estimate for this IgG-only test. All 16 initially negative participants who had follow-up testing became positive during recovery.

PCR detects the bacterium's DNA and is most useful early in the illness. Antibody testing becomes more informative as the immune response develops. There isn't a universal day when one test replaces the other, and neither an early negative antibody result nor a negative PCR result reliably excludes infection.

Positive Results in Healthy People

Detectable IgG doesn't necessarily mean you're currently ill. In one study, about 16% of healthy blood donors in western Norway had reactive serum IgG by IFA. A separate study of farmers, forestry workers, and veterinarians in eastern Poland found reactive IgG in about 10%. These are findings from particular populations and assays, not estimates for everyone who spends time outdoors.

Some positive results reflect previous infection, including infection that caused few or no symptoms. Others reflect antibodies that react with related organisms. The height of a single titer cannot reliably date an infection or prove that it explains your current symptoms.

When Results Can Be Misleading

• Early sampling: antibodies are frequently undetectable during the first week of illness.
• Persistent antibodies: IgG can remain detectable for months or years after recovery. Persistent positivity alone doesn't establish ongoing infection.
• Related organisms: antibodies against Ehrlichia and other Anaplasma species can react in this assay. Positive results for both organisms don't necessarily mean you have both infections.
• Differences between assays: the bacterial material on the slides, dilution procedures, and interpretation under the microscope can affect the reported titer. Laboratories may use different slide sources and starting dilutions, so results from different laboratories aren't directly interchangeable.
• A weakened antibody response: conditions or treatments that impair antibody production can make a negative result less reassuring.
• Sample quality: substantial breakdown of blood cells, excess fat in the specimen, or contamination can make a sample unsuitable for testing.

Small Japanese studies found that some people's antibodies reacted differently depending on the cells used to grow the bacteria for the test slides. This helps explain why two IFA methods can disagree even when they are looking for antibodies to the same organism.

How Antibodies Change After Recovery

Antibody persistence varies considerably. The Korean cohort showed an IgG peak around four weeks followed by declining reactivity, but long-term follow-up was limited. Other human studies document antibodies persisting for years. A falling titer doesn't prove that treatment worked, and a persistent titer doesn't prove that treatment failed.

There is no established antibody titer that guarantees protection against another infection. If you've had anaplasmosis before, an old positive result can make a new illness harder to interpret.