Description of the blood test:

The Bartonella henselae IgM test is a specialized serological assay used to detect an early immune response to the bacterium responsible for Cat Scratch Disease (CSD). Bartonella henselae is typically transmitted to humans through the scratch or bite of a cat (especially kittens) or, less commonly, by cat fleas. IgM antibodies are the first to be produced by the immune system, serving as a primary marker for a current or very recent infection. Identifying these antibodies is essential for diagnosing the cause of localized lymph node swelling and fever, particularly when there is a history of contact with felines.

What does the analysis represent?

• Acute Infection Marker: IgM antibodies generally appear within a few days to a week after symptoms begin and may remain detectable for up to 3 months.
• Diagnostic Target: Used to differentiate CSD from other causes of lymphadenopathy (swollen lymph nodes), such as sarcoidosis, tuberculosis, or lymphoma.
• Pathogen Specificity: Specifically targets Bartonella henselae, the most common species associated with human infection.
• Performance: Indirect Fluorescent Antibody (IFA) or Enzyme Immunoassay (ELISA) performed on a venous blood sample.

Recommendations for the test (General)

• Time: It is recommended to have the blood drawn during the morning hours.
• Fasting: A food intake pause of 4–8 hours is preferred.
• Clinical History: Always inform your doctor about recent cat scratches, bites, or exposure to fleas, as this is vital for clinical correlation.
• Important: If the test is performed too early (within the first 48 hours of symptoms), the IgM may not yet be detectable. A repeat test 10–14 days later may be required if suspicion remains high.

What can affect the results?

• Cross-reactivity: False-positive results may occur due to cross-reacting antibodies from other infections, such as Bartonella quintana (Trench fever), Coxiella burnetii (Q fever), or even Epstein-Barr Virus (EBV).
• Immune Status: Immunocompromised individuals (e.g., those with HIV/AIDS) may produce a weak or absent IgM response, even with a severe infection (bacillary angiomatosis).
• Antibiotic Use: Prior treatment with certain antibiotics may decrease the antibody titer, potentially leading to a negative result.

When to take the test?

• Recommendations: Prescribed for patients exhibiting painfully swollen lymph nodes (usually near the site of a scratch on the arms, neck, or head), persistent fever, fatigue, or headache following contact with a cat.
• Preparation: Standard preparation for serological testing; no specific dietary restrictions beyond fasting.

How to interpret the results?

The interpretation of data is the exclusive responsibility of an infectious disease specialist. A positive IgM result is a strong indicator of an acute Bartonella infection. However, because these antibodies can sometimes persist or cross-react, the result must be interpreted alongside Bartonella IgG levels and clinical findings. Only an expert can determine if the infection is limited to the lymph nodes or if it has affected other organs (such as the liver, spleen, or eyes) and prescribe the appropriate course of treatment.

Possible further investigations

• Bartonella henselae IgG: To determine if the infection is progressing or if there is long-term immune memory.
• PCR for Bartonella: A highly specific molecular test to detect the bacterial DNA directly from a blood sample or lymph node biopsy.
• Ultrasound of Lymph Nodes: To evaluate the structure and size of the affected nodes.
• Consultation with an Infectious Disease Specialist: To confirm the diagnosis and manage potential complications.

When does the next step make sense?

A positive IgM result suggests an active battle between your immune system and the Bartonella bacteria. The next step is a consultation with a specialist to ensure the infection is properly monitored and to determine if antibiotic therapy is necessary to speed up recovery and prevent systemic spread.

👉 If necessary, you can discuss the test results with our specialized — infectious disease specialist Dermatologist (Doctors – TAMC).