Can Babesia Tests Be Negative? When Diagnosis Is Clinical
Babesia can sometimes be missed when parasite levels in the blood are low
A 2024 case was initially missed on blood-smear examination before PCR confirmed Babesia microti
Symptoms, exposure history, timing, and the type of test all matter when evaluating suspected babesiosis
Can Babesia tests be negative even when babesiosis is still suspected? Yes. Blood-smear examination and PCR are important tests for diagnosing acute babesiosis, but testing does not always provide an immediate answer.
A 2024 case illustrates the problem particularly well. A 75-year-old man became ill after spending approximately three weeks at a lake house in Maine. His initial blood-smear examination was reported as negative for malaria parasites. After clinicians and laboratory specialists reconsidered his travel history, the smear was reviewed again and scant ring forms were identified. PCR and DNA sequencing ultimately confirmed Babesia microti, even though his parasitemia was only 0.2%.1
The case does not mean that blood smears are unreliable. Instead, it demonstrates why a single initially unrevealing smear may not always settle the diagnostic question when the clinical and exposure history continues to suggest babesiosis.
For a broader overview of the infection, see Babesia Symptoms and Treatment.
How Is Babesiosis Diagnosed?
Babesia parasites infect red blood cells. During acute infection, clinicians can look directly for the parasite by examining stained blood under a microscope.
PCR takes a different approach. Rather than visually identifying organisms, PCR looks for Babesia DNA in the blood.
Current guidelines recommend peripheral blood-smear examination or PCR for confirmation of acute babesiosis.2,3 Antibody testing may provide additional information, but a single positive antibody result cannot by itself distinguish active infection from previous exposure.
The diagnostic challenge is that the usefulness of these tests can depend on parasite burden, timing, species, laboratory technique, and the clinical circumstances in which testing is performed.
How Was Babesiosis Initially Missed in the 2024 Case?
The patient in the 2024 report had traveled from South Africa to the United States and spent approximately three weeks at a lake house in Maine, where he participated in outdoor activities. After returning home, he developed profound fatigue followed by fever, muscle aches, joint pains, headache, gastrointestinal symptoms, and other systemic complaints.1
He did not remember a tick bite.
Laboratory testing showed anemia, marked thrombocytopenia, elevated liver enzymes, inflammation, and evidence of kidney involvement. Malaria was initially among the diagnostic considerations because the patient was being evaluated in South Africa and Babesia can resemble malaria microscopically.1
The initial thin blood smear was reported as negative for malaria parasites, and malaria antigen testing was also negative.
But the epidemiologic history changed the interpretation of the case.
Once laboratory specialists considered the possibility that the infection had been acquired during the patient’s stay in Maine, the smear was reviewed again. This time, scant intraerythrocytic ring forms were recognized and babesiosis was considered.1
PCR followed by Sanger sequencing confirmed B. microti.
Why Was the 0.2% Parasitemia Important?
Only 0.2% of the patient’s red blood cells were reported to be infected.1
That low parasite burden helps explain why the case is relevant to patients asking whether Babesia can be missed on a blood smear.
A blood smear depends on actually seeing infected red blood cells under a microscope. When infected cells are sparse, identifying the organism can require examination of many microscopic fields and careful interpretation.
Current IDSA guidance notes that PCR is more sensitive than blood-smear examination when the parasite burden is low. The guideline also recommends examining multiple thin-smear fields and states that PCR should be performed when smears are negative but babesiosis is still suspected.3
CDC similarly notes that multiple smears may sometimes need to be examined and recommends manual review of the peripheral blood smear when babesiosis is being considered.2
Does a Negative Blood Smear Rule Out Babesiosis?
Not necessarily.
A negative smear lowers the evidence for active parasitemia, but the result needs to be interpreted in context.
If acute babesiosis remains clinically plausible despite a negative smear, PCR may provide additional diagnostic information because it can detect parasite DNA when the number of infected red blood cells is too low for easy microscopic recognition.3
The 2024 case is particularly instructive because the organisms were not absent. They were present in small numbers and were recognized only after the smear was reconsidered in light of the patient’s exposure history.1
This distinction is important. The case should not be interpreted as evidence that every negative smear represents missed babesiosis. Instead, it shows that laboratory results and clinical probability sometimes need to be reconsidered together.
Why Exposure History Can Change the Diagnostic Picture
The patient’s travel history was central to the diagnosis.
B. microti is transmitted primarily by Ixodes scapularis ticks in the northeastern and upper midwestern United States. Maine therefore represented an epidemiologically plausible location for exposure.
Once clinicians recognized that connection, the laboratory interpretation changed.
The authors emphasized communication between clinicians and laboratory personnel because the same microscopic ring forms may initially suggest malaria in one epidemiologic setting and babesiosis in another.1
Where a patient has been can sometimes be as important as what appears on the first laboratory report.
Do You Need to Remember a Tick Bite to Have Babesiosis?
No. The patient in the 2024 case did not recall a tick bite despite having PCR-confirmed B. microti infection.1
Blacklegged ticks can be small, and a bite may go unnoticed. Therefore, the absence of a remembered tick bite should not automatically exclude babesiosis when the geographic exposure, symptoms, and laboratory findings are otherwise compatible.
This is especially relevant when evaluating patients who have spent time hiking, gardening, camping, working outdoors, or participating in other activities in areas where infected ticks are present.
What Laboratory Findings Can Raise Suspicion for Babesiosis?
Babesiosis can produce laboratory abnormalities related to infection of red blood cells and systemic illness.
Common findings can include:
- Anemia
- Thrombocytopenia, or a low platelet count
- Elevated liver enzymes
- Elevated bilirubin or lactate dehydrogenase when hemolysis is present
- Reduced haptoglobin
- Protein or hemoglobin in the urine
- Abnormal kidney function in more severe illness
None of these findings is specific to Babesia. However, the combination of compatible symptoms, laboratory abnormalities, and plausible tick exposure may increase clinical suspicion and help determine whether additional Babesia testing is appropriate.2,3
What Did the Earlier Indiana Investigation Find?
An earlier CDC and Indiana State Department of Health investigation illustrates a different diagnostic problem.
In 2016, investigators evaluated 14 patients who had previously been clinically diagnosed with Lyme disease and babesiosis. CDC tested specimens using Giemsa-stained blood smears, PCR, and indirect fluorescent antibody testing for B. microti.4
The specimens were obtained relatively late. They were collected a median of 172 days after reported illness onset, and most patients had already received multiple antimicrobial agents.4
All blood smears and PCR tests for Babesia were negative. Two patients had B. microti IFA titers of 1:64, while the remaining Babesia testing was negative.4
The investigators concluded that their laboratory investigation did not demonstrate a cluster of babesiosis or Lyme disease among these patients.
However, the study had an important limitation: investigators did not conduct patient interviews or review the patients’ medical charts. Clinical information came from specimen-submission forms, and exposure and travel histories were not provided.4
For that reason, the investigation provides useful information about what the later laboratory testing showed, but it cannot reconstruct all of the clinical information available when the original diagnoses were made.
Surveillance Criteria and Clinical Diagnosis Are Not the Same Thing
Public-health surveillance definitions are designed to classify and count cases consistently across jurisdictions. They are not designed to replace individualized medical assessment.
The current CDC babesiosis surveillance definition explicitly states that surveillance case definitions are not intended to be used by healthcare providers to make a clinical diagnosis or determine an individual patient’s healthcare needs.5
Current surveillance criteria recognize microscopy, molecular testing, changes in antibody titers, and certain antibody thresholds, depending on the type and timing of the evidence.5
This distinction is important when reading older reports such as the 2018 Indiana investigation. Surveillance definitions can also change over time, so historical surveillance thresholds should not automatically be presented as current clinical diagnostic rules.
What Should Happen When the Smear Is Negative but Babesiosis Is Still Suspected?
A negative smear should be interpreted alongside the patient’s symptoms, exposure history, routine laboratory abnormalities, timing of testing, and overall likelihood of babesiosis.
For suspected acute babesiosis, current IDSA guidance recommends PCR when blood smears are negative but clinical suspicion remains.3
CDC also emphasizes manual smear review and notes that multiple smears may sometimes be necessary.2
This approach avoids two opposite errors: assuming that every negative test completely excludes babesiosis, or assuming that compatible symptoms automatically establish the diagnosis.
Clinical judgment is most useful when it guides appropriate testing and interpretation rather than treating either symptoms or laboratory results in isolation.
Frequently Asked Questions
Can you have Babesia with a negative blood smear?
Yes. Babesia may occasionally be missed on an initial blood smear, particularly when parasite levels are low. If acute babesiosis remains suspected despite a negative smear, PCR can provide additional diagnostic information.
Can PCR detect Babesia when a blood smear is negative?
Yes. PCR is generally more sensitive than blood-smear examination when parasite burden is low. Current guidelines recommend considering PCR when smears are negative but babesiosis remains clinically suspected.
How low was the parasitemia in the 2024 Babesia case?
The patient’s parasitemia was only 0.2%. His initial smear examination did not identify the diagnosis, but subsequent smear review showed scant ring forms and PCR with DNA sequencing confirmed Babesia microti.
Do you need to remember a tick bite to have babesiosis?
No. Patients may not notice or remember a tick bite. The patient in the 2024 case had confirmed Babesia microti infection but did not recall being bitten by a tick.
Does a negative Babesia test prove that symptoms are not caused by babesiosis?
A negative result needs to be interpreted according to the test used, timing, parasite burden, exposure history, and clinical presentation. A negative test does not establish babesiosis, but an initially negative smear may not always end the evaluation when acute infection remains clinically plausible.
Clinical Takeaway
Babesia testing is an important part of diagnosing acute babesiosis, but laboratory results need to be interpreted within the clinical setting.
The 2024 case provides a concrete example. A patient with compatible illness and exposure in Maine initially had a smear that did not identify the diagnosis. Once his travel history prompted another review, scant ring forms were recognized, and PCR with sequencing confirmed B. microti despite parasitemia of only 0.2%.1
A negative initial smear does not prove that babesiosis is present, but it may not completely settle the question when the exposure history and clinical findings remain compelling.
When acute babesiosis remains suspected after a negative smear, additional smear review and PCR may help clarify the diagnosis.
Related Articles
Babesia Testing: Why False Negatives Can Delay Treatment
Babesia Negative Tests: Why Treatment May Still Be Needed
Babesia Symptoms: Why This Tick-Borne Infection Is Often Missed
Babesia: Symptoms, Testing, Treatment, and Lyme Disease
Case Report: Various Clinical Presentations of Babesia
References
- Mac Donald JW, Frean JA, Ratabane JM, Moodley B, Mannaru K, Holz GE. A case of babesiosis in a returning traveller. S Afr J Infect Dis. 2024;39(1):588. doi:10.4102/sajid.v39i1.588. PMID: 38628426; PMCID: PMC11019078.
- Centers for Disease Control and Prevention. Clinical Overview of Babesiosis. Updated February 13, 2024.
- Krause PJ, Auwaerter PG, Bannuru RR, et al. Clinical Practice Guidelines by the Infectious Diseases Society of America (IDSA): 2020 Guideline on Diagnosis and Management of Babesiosis. Clin Infect Dis. 2021;72(2):e49-e64. doi:10.1093/cid/ciaa1216.
- Brown JA, Allman R, Herwaldt BL, et al. Notes from the Field: Reference Laboratory Investigation of Patients with Clinically Diagnosed Lyme Disease and Babesiosis — Indiana, 2016. MMWR Morb Mortal Wkly Rep. 2018;67(41):1160-1161. doi:10.15585/mmwr.mm6741a6.
- Centers for Disease Control and Prevention. Babesiosis 2025 Case Definition. National Notifiable Diseases Surveillance System.
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
