What this test measures
Galactomannan (GM) is a polysaccharide cell-wall component of Aspergillus species. It is shed into the bloodstream during invasive aspergillosis as the fungus grows in tissue. The serum GM enzyme immunoassay (Platelia, Bio-Rad) reports a galactomannan index (GMI) — the ratio of patient signal to a positive control. A GMI of ≥ 0.5 in serum is considered positive in most modern guidelines.
The assay can also be performed on bronchoalveolar lavage (BAL) fluid, where it is more sensitive for invasive pulmonary aspergillosis — a BAL GMI ≥ 1.0 is considered positive. Serial serum monitoring (twice weekly) in high-risk patients can detect infection days to weeks before clinical or radiological signs appear.
Why it matters
Invasive aspergillosis is a feared complication in immunocompromised patients — prolonged neutropenia (acute leukaemia, stem-cell transplant), high-dose steroids, advanced HIV, severe lung disease, and ICU patients with severe influenza or COVID-19. Mortality remains high (30–50%) and outcome depends heavily on early diagnosis and prompt antifungal therapy (voriconazole, isavuconazole).
Indian haematology and transplant centres now use twice-weekly serum galactomannan screening as part of standard care for high-risk patients. A rising GMI over consecutive samples is more meaningful than a single value — it triggers CT chest, BAL, and empirical antifungal therapy. The test is also used to monitor response: a falling GMI on antifungal treatment indicates response, while a rising GMI suggests treatment failure.
How to prepare
No fasting required. Continue all medications. Tell the lab about any beta-lactam antibiotics — piperacillin-tazobactam (older formulations) and some other antibiotics historically caused false-positive results, though modern formulations are largely free of this problem. Also disclose any recent IV fluids containing plasma-Lyte, nutritional supplements with soybean-derived components, and recent administration of intravenous immunoglobulin.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Galactomannan Index (Serum) (GMI (ratio))[1][2][3] | < 0.5 (negative) | < 0.5: negative — supports absence of invasive aspergillosis at the time of sampling. A single negative result does not rule out infection in a high-risk patient — repeat sampling twice weekly is standard. | 0.5 – 0.7: borderline / weak positive; repeat sample in 24–48 hours. ≥ 0.7 on a single sample, or two consecutive ≥ 0.5 samples: positive — supports probable invasive aspergillosis; proceed to CT chest, BAL, and consider empirical antifungal therapy. A rising trend across samples is the most reliable diagnostic signal. |
Galactomannan interpretation by specimen
| Specimen | Positive cut-off | Performance |
|---|---|---|
| Serum | GMI ≥ 0.5 | Sensitivity 60–80% in neutropenic patients |
| Bronchoalveolar lavage (BAL) | GMI ≥ 1.0 | Sensitivity > 85% for invasive pulmonary aspergillosis |
| Cerebrospinal fluid | Any positive — interpret carefully | For suspected CNS aspergillosis |
Frequently asked questions
Who should be tested with galactomannan?
Patients at high risk of invasive aspergillosis: prolonged neutropenia after chemotherapy, allogeneic stem-cell or solid-organ transplant recipients, advanced HIV, high-dose steroid therapy, severe lung disease (COPD with steroids), and ICU patients with severe influenza or COVID-19 pneumonia.
How often is the test repeated?
Twice-weekly serum testing during high-risk periods (e.g. duration of neutropenia, intensive phase of chemotherapy). A rising trend across samples is more reliable than a single positive result.
What can cause a false-positive galactomannan?
Historically, piperacillin-tazobactam was a common cause; modern formulations have largely resolved this. Other causes: certain plasma-protein fractions, IV immunoglobulin, plasma-Lyte solutions, soya-bean-based nutritional supplements, and rarely cross-reaction with other fungi (Penicillium, Histoplasma, Fusarium).
Can the test miss invasive aspergillosis?
Yes — particularly in non-neutropenic patients (e.g. COPD, ICU), in patients on prophylactic antifungals (which lower the GMI), and in very early or contained infection. Diagnostic suspicion should not rest on the test alone; CT chest and BAL are important when clinical suspicion is high.
Is BAL galactomannan better than serum?
For invasive pulmonary aspergillosis, BAL GM has higher sensitivity than serum (especially in non-neutropenic patients). A BAL GMI ≥ 1.0 is highly supportive. Serum is used because it is non-invasive and easy to repeat.
How long does it take to get the result?
Most large NABL labs in India turn around galactomannan in 24–48 hours. In suspected severe invasive aspergillosis, empirical antifungal therapy is started before the result returns, and is then reviewed when results, imaging, and BAL are available.
Can it monitor treatment response?
Yes — serial GMI values are a useful adjunct. A falling GMI on antifungal treatment supports response; a rising GMI suggests treatment failure, breakthrough infection, or resistant Aspergillus.
Related Immunology tests
Tests commonly ordered alongside ASPERGILLUS GALACTOMANNAN ANTIGEN, or that help interpret an unexpected result.
Sources & references
- NCBI StatPearls — Aspergillosis · accessed 2026-05-30T00:00:00.000Z
- IDSA — Practice Guidelines for Diagnosis and Management of Aspergillosis · accessed 2026-05-30T00:00:00.000Z
- Mayo Clinic Labs — Aspergillus Galactomannan Antigen · accessed 2026-05-30T00:00:00.000Z
- CDC — Aspergillosis · accessed 2026-05-30T00:00:00.000Z
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