Can an Anaplasmosis Blood Test Be Negative Early in the Illness?
Anaplasmosis antibody tests may be negative during the first week
Whole-blood PCR can help detect infection before antibodies appear
Treatment should not be delayed when anaplasmosis is strongly suspected
Can an anaplasmosis blood test be negative early in the illness? Yes. Antibody tests for Anaplasma phagocytophilum are frequently negative during the first week of illness because the immune system may not yet have produced detectable antibody levels.1
This does not mean that all blood tests for anaplasmosis are unreliable early. Polymerase chain reaction (PCR) testing of whole blood can be particularly useful during the first week because it looks for genetic material from the organism rather than relying on the patient’s antibody response.1
A 2026 case illustrates this diagnostic problem. A 77-year-old man developed fever, lethargy, gastrointestinal symptoms, confusion, visual hallucinations, thrombocytopenia, elevated liver enzymes, and other laboratory abnormalities after traveling from Minnesota to Japan. Initial antibody testing for A. phagocytophilum was negative, but PCR testing of whole blood was positive.2
The patient was treated with doxycycline and became afebrile the following day, with improvement in his mental status. The case demonstrates why an early negative antibody test should not automatically exclude anaplasmosis when the clinical picture and exposure history remain concerning.2
Why can anaplasmosis antibody tests be negative early?
Serologic testing does not detect the bacterium itself. Instead, it measures antibodies produced by the immune system in response to infection.
Those antibodies take time to develop. According to the Centers for Disease Control and Prevention (CDC), IgG antibody titers measured by indirect fluorescent antibody testing, or IFA, are frequently negative during the first week of anaplasmosis.1
For that reason, a single negative antibody test obtained early in the illness cannot reliably exclude the infection. The timing of the blood sample matters because a patient may already be acutely ill before a measurable antibody response has developed.
2026 case: Antibody tests were negative but PCR was positive
Ko and colleagues reported the case of a 77-year-old man from Bemidji, Minnesota, who became ill while traveling in Japan. Before leaving the United States, a possible tick was reportedly found in his underwear.2
Several days later, he developed fever, lethargy, loss of appetite, vomiting, diarrhea, and eventually significant neurological symptoms. His illness included confusion, disorientation, visual hallucinations, and irritability.2
Laboratory testing revealed severe thrombocytopenia, elevated liver enzymes, increased creatine kinase, and markedly elevated inflammatory markers. No morulae were identified on the peripheral blood smear.2
Testing for IgM and IgG antibodies against A. phagocytophilum by indirect immunofluorescence was negative on admission. However, PCR testing of whole blood detected A. phagocytophilum. Sequence analysis of 16S rRNA, groEL, and p44 fragments showed 100% identity with reference A. phagocytophilum sequences.2
The patient received doxycycline 100 mg twice daily along with empiric cefepime. He became afebrile the next day, his mental status improved, and cefepime was discontinued. He was discharged in stable condition on hospital day 5.2
The case is important because the negative antibody result could have been falsely reassuring if clinicians had relied on serology alone.
Is PCR better than antibody testing early in anaplasmosis?
PCR and antibody testing detect different evidence of infection, and their usefulness changes with the timing of the illness.
PCR looks for genetic material from A. phagocytophilum. The CDC recommends PCR testing on whole blood and states that the method is most sensitive during the first week of illness.1
Serology measures the patient’s immune response. Because antibody production takes time, serology is frequently negative during the first week and is best interpreted using appropriately timed acute and convalescent samples.1
This creates an important diagnostic window: a patient may already be symptomatic, the antibody test may still be negative, and whole-blood PCR may detect the organism.
However, PCR is not perfect. A negative PCR result does not completely exclude anaplasmosis when clinical suspicion remains high.1
Why does the timing of PCR testing matter?
PCR is most useful early in the course of anaplasmosis, but its sensitivity can decline after effective antibiotic treatment begins.
The CDC reports that the sensitivity of whole-blood PCR decreases within approximately 48 hours after administration of appropriate antibiotics.1 The 2026 case report also emphasizes the diagnostic value of molecular testing during the early phase of infection, when serology may remain negative.2
When possible, blood for PCR should therefore be obtained promptly. However, obtaining diagnostic testing should not create a delay in treatment when anaplasmosis is strongly suspected.
Can a later antibody test confirm anaplasmosis?
Yes. Paired antibody testing can help confirm a recent infection after an initially negative acute test.
The CDC recommends IgG IFA testing using paired acute and convalescent serum samples. The acute specimen should be collected during the first two weeks of illness, with the convalescent specimen collected 2 to 10 weeks later. A fourfold increase in IgG antibody titer provides evidence of seroconversion.1
This helps explain an apparently contradictory situation: a patient’s first antibody test can be negative even though the patient has anaplasmosis, while a later specimen may demonstrate the antibody response that was not yet detectable during the acute illness.
IgM testing should not be relied upon as evidence of recent anaplasmosis because the CDC does not consider IgM a reliable method for diagnosing the infection.1
Does a negative PCR rule out anaplasmosis?
No. Although a positive PCR provides clear evidence of active infection, a negative PCR does not necessarily exclude anaplasmosis.1
The likelihood of detecting the organism depends partly on when the specimen was collected and whether effective antibiotics had already been started.
This is one reason anaplasmosis remains partly a clinical diagnosis. Symptoms, laboratory abnormalities, possible tick exposure, geography, travel history, timing of testing, PCR results, and serology may all contribute to the assessment.
Can routine blood tests also be normal early in anaplasmosis?
Anaplasmosis commonly produces thrombocytopenia, leukopenia, and elevated liver enzymes. However, these abnormalities are not necessarily present in every patient or at every point in the illness.3
The CDC notes that normal laboratory findings do not rule out possible infection.3 This is different from having a negative antibody or PCR test, but both issues illustrate why the timing and type of laboratory testing matter.
For more on this diagnostic problem, see Anaplasmosis Without Classic Laboratory Findings.
Should treatment wait for a positive anaplasmosis test?
No. The CDC advises clinicians not to delay or withhold treatment while waiting for laboratory results when anaplasmosis is suspected. Treatment also should not be withheld solely because an initial laboratory test is negative.1
Early recognition is important because anaplasmosis can occasionally progress to severe illness. Doxycycline is the treatment of choice for suspected anaplasmosis, and diagnostic testing should support clinical decision-making rather than create a delay in appropriate treatment.1
Frequently Asked Questions
Can you have anaplasmosis with a negative antibody test?
Yes. IgG antibody testing is frequently negative during the first week of illness because detectable antibodies may not have developed yet. A negative early antibody test therefore does not rule out anaplasmosis.
What is the best test for anaplasmosis early in the illness?
PCR performed on whole blood is particularly useful early in the illness. The CDC states that whole-blood PCR is most sensitive during the first week of illness.
Can an anaplasmosis PCR test be negative?
Yes. A negative PCR does not completely rule out anaplasmosis. The likelihood of detection depends partly on the timing of testing, and PCR sensitivity decreases after effective antibiotic treatment begins.
When do anaplasmosis antibodies become positive?
Antibodies may not be detectable during the first week. Serologic confirmation is best supported by paired acute and convalescent IgG IFA samples demonstrating a fourfold rise in antibody titer.
Should doxycycline be delayed until anaplasmosis testing is positive?
No. When anaplasmosis is clinically suspected, treatment should be started without waiting for confirmatory laboratory testing. An initially negative test should not by itself be used to withhold treatment.
Clinical Takeaway
A negative early antibody test does not rule out anaplasmosis. Antibodies may not yet be detectable during the first week of illness, while whole-blood PCR can provide useful evidence of infection during this early diagnostic window.
The 2026 case provides a clear example: antibody testing for A. phagocytophilum was negative at admission while whole-blood PCR was positive. The patient improved rapidly after doxycycline was started.2
When anaplasmosis is suspected, the type and timing of testing should be considered rather than interpreting an early negative result in isolation.
Related Articles
For additional information about anaplasmosis, neurological complications, and tick-borne coinfections, see:
Anaplasmosis Can Cause Encephalitis: A Rare but Serious Neurologic Complication
Anaplasmosis Neurologic Symptoms and Brain Involvement
Anaplasmosis and Lyme Disease: Symptoms, Lab Findings, and Treatment
Lyme Disease Coinfections: Babesia, Bartonella & More
References
- Centers for Disease Control and Prevention. Clinical Testing and Diagnosis for Anaplasmosis. CDC. Updated May 15, 2024.
- Ko S, Sakurai A, Taira M, Katano H, Maeda K, Suzuki T, Ohmagari N. An imported case of human granulocytic anaplasmosis presenting with altered mental status: A case report. IDCases. 2026;44:e02626. doi:10.1016/j.idcr.2026.e02626.
- Centers for Disease Control and Prevention. Clinical Signs and Symptoms of Anaplasmosis. CDC. Updated May 15, 2024.
Dr. Daniel Cameron, MD, MPH
Lyme disease clinician with over 30 years of experience and past president of ILADS.
Symptoms • Testing • Coinfections • Recovery • Pediatric • Prevention
