Babesia FISH
Babesia FISH uses fluorescent-labelled nucleic-acid probes to detect intraerythrocytic Babesia parasites in whole blood. It is more sensitive than conventional microscopy, does not require expert identification of ring forms, and is ordered during acute febrile illness in tick-endemic areas to confirm active babesiosis, evaluate co-infection with Lyme, and document clearance after therapy.
1
Biomarkers
Blood (whole blood)
Sample
5–7 business days
Turnaround
No
Fasting
What is the Babesia FISH test?
Babesia FISH is a direct detection method that identifies intraerythrocytic Babesia parasites through fluorescent-labelled nucleic-acid probes targeting the organism's ribosomal RNA. It is ordered when acute babesiosis is suspected — particularly in endemic tick-exposure regions — and serology or blood-film microscopy has been inconclusive or unavailable. The test is most valuable during the first few weeks of symptomatic illness, when parasitaemia is detectable and before antibody responses have fully matured. Because co-infection with Borrelia burgdorferi is common in Ixodes-endemic areas, practitioners often order Babesia FISH alongside Lyme serology and PCR when the clinical picture includes fever, haemolytic anaemia, or an atypical response to tick-borne illness. FISH offers higher sensitivity than Giemsa-stained smear, does not depend on observer skill in identifying ring forms, and provides a definitive yes-or-no result during the parasitaemic window.
What does the Babesia FISH test measure?
Babesia are intraerythrocytic protozoan parasites transmitted by Ixodes ticks, the same vector that transmits Borrelia burgdorferi. The two predominant species in North America are B. microti and, less commonly, B. duncani. Once inoculated, sporozoites invade red cells, replicate asexually, and lyse the host erythrocyte to release merozoites that infect fresh cells. Parasitaemia is typically low and intermittent, peaking during the acute febrile phase and then fluctuating or clearing over weeks to months in immunocompetent hosts, but may persist in asplenic or immunocompromised patients.
Fluorescence in situ hybridisation uses species-specific DNA probes labelled with fluorophores to bind Babesia ribosomal RNA within infected erythrocytes, making even low-level parasitaemia visible under fluorescence microscopy. FISH is more sensitive than conventional Giemsa-stained blood film and does not require the microscopist to distinguish ring forms from stain artefact or Plasmodium. It is particularly useful during the acute presentation when parasitaemia is highest, in co-infected patients where symptoms overlap, and in follow-up of treated cases to document clearance.
Why is the Babesia FISH test ordered?
- Acute febrile illness with recent tick exposure in Ixodes-endemic regions (northeastern and upper midwestern United States), particularly when accompanied by haemolytic anaemia, thrombocytopaenia, or elevated lactate dehydrogenase
- Suspected co-infection in patients with known or suspected Lyme disease whose symptoms include high fever, night sweats, or signs of haemolysis
- Asplenic or immunocompromised patients with fever and tick exposure, at higher risk of severe or persistent parasitaemia
- Evaluation of fever in a patient who has received a blood transfusion in recent weeks, as transfusion-transmitted babesiosis is increasingly recognised
- Follow-up testing to document clearance of parasitaemia after treatment, especially in asplenic patients or when symptoms persist
- Clarification of inconclusive blood-film microscopy or when expertise in Giemsa-stain interpretation is not locally available
- Investigation of unexplained haemolytic anaemia and splenomegaly in an endemic area
Sample collection and turnaround
Sample type
Blood (whole blood)
Fasting required
No
Collection method
Venipuncture at patient service centre
Turnaround
5–7 business days
Collection notes
Phlebotomy at a draw site that ships to IgeneX; the laboratory will provide a collection kit with EDTA tubes and prepaid shipping. Because FISH targets ribosomal RNA, prompt processing is important — follow IgeneX instructions for ambient versus refrigerated shipment and same-day or next-day dispatch. Collect during the acute febrile phase whenever possible, before antimicrobial therapy is initiated.
Specimen requirements
Whole blood in EDTA (typically 3–5 mL); consult the IgeneX kit insert for exact volume. Ship at ambient or refrigerated temperature per kit instructions and ensure arrival within the timeframe specified to preserve RNA integrity.
What can affect Babesia FISH results?
Parasitaemia in immunocompetent hosts peaks early in the acute illness and declines thereafter; samples drawn during convalescence or after treatment may miss low or cleared parasitaemia, so a negative result does not exclude prior infection.
Treatment with atovaquone–azithromycin or clindamycin–quinine rapidly reduces parasite load; FISH performed after even a few days of therapy may be falsely negative.
Asplenic and immunocompromised patients sustain higher and more prolonged parasitaemia, so FISH remains positive for longer periods and sensitivity is higher in this population.
FISH targets ribosomal RNA, which is labile; delays in processing or improper specimen handling can degrade signal. Follow the laboratory's collection and transport instructions strictly.
FISH probes are designed for B. microti and may have variable sensitivity for B. duncani or other rarer species. Confirm with the laboratory which species are covered by the assay.
Very low-level parasitaemia — below approximately 0.001–0.01% — may fall beneath the practical sensitivity of FISH, so a negative result in early or resolving infection should be correlated with serology and clinical context.
What do Babesia FISH results mean?
Elevated
Not applicable — this is a qualitative presence/absence assay.
Normal
Negative: no fluorescent-labelled parasites detected. This result indicates that Babesia organisms were not visualised in the sample submitted. In the appropriate clinical window (acute febrile phase) a negative FISH argues against active babesiosis, but it does not exclude infection if the sample was drawn late in the illness, after treatment, or during a trough in parasitaemia. Negative FISH should be interpreted with Babesia serology and the overall clinical picture.
Reduced
Not applicable.
Note: FISH is most sensitive during the acute parasitaemic phase; a negative result later in the illness or post-treatment does not exclude babesiosis. Positive FISH confirms active infection and, in endemic regions with co-circulation of Borrelia, should prompt evaluation for Lyme co-infection. The species detected (B. microti, B. duncani, or other) may not be specified in the report — confirm probe targets with IgeneX if species identification matters to treatment choice or epidemiology. FISH does not quantify parasitaemia; if a parasite load estimate is needed, request a quantitative blood-film count or PCR if available.
Frequently asked questions
Babesia FISH uses fluorescent DNA probes that bind to Babesia ribosomal RNA inside infected red blood cells, making parasites visible under fluorescence microscopy. It detects active parasitaemia during acute infection and is more sensitive than conventional Giemsa-stained blood films.
How to access the Babesia FISH test
The Babesia FISH reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: October 7, 2026
Last reviewed: October 7, 2026
- Krause PJ, et al. Clinical practice guidelines by the Infectious Diseases Society of America (IDSA): diagnosis and management of babesiosis. Clin Infect Dis. (2021)
- Vannier EG, Diuk-Wasser MA, Ben Mamoun C, Krause PJ. Babesiosis. Infect Dis Clin North Am. (2015)
- Leiby DA. Transfusion-transmitted Babesia spp.: bull's-eye on Babesia microti. Clin Microbiol Rev. (2011)
- Persing DH, et al. Detection of Babesia microti by polymerase chain reaction. J Clin Microbiol. (1992)
- Wormser GP, et al. The clinical assessment, treatment, and prevention of Lyme disease, human granulocytic anaplasmosis, and babesiosis: IDSA clinical practice guidelines. (2006)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.