TB or Not TB?

Making Sense of QuantiFERON Conundrums

By Dr Linda Dreyer
Published June 2026

Interferon-gamma release assays (IGRAs) are in vitro blood tests that measure cell-mediated immune responses to antigens specific to the Mycobacterium tuberculosis complex. In Australia, QuantiFERON-TB Gold Plus (QFTPlus) is the most widely used IGRA for the diagnosis of latent tuberculosis infection (LTBI).

LTBI represents persistent immune sensitisation to M. tuberculosis without clinical or radiological evidence of active disease. Individuals with LTBI are asymptomatic and not infectious, but carry a lifetime risk of progression to active tuberculosis (TB), estimated at approximately 5–10%, with the highest risk occurring within the first two years following infection.

IGRAs are not diagnostic of active TB and must always be interpreted in conjunction with clinical assessment, exposure history, and, where indicated, imaging and microbiology.

Understanding QuantiFERON results

QuantiFERON-TB Gold Plus measures interferon-gamma (IFN-γ) released by sensitised T lymphocytes following stimulation with M. tuberculosis-specific antigens.

The assay consists of four blood collection tubes:

Nil control – measures background IFN-γ
Mitogen control – positive control assessing immune competence
TB1 antigen tube – primarily stimulates CD4+ T-helper lymphocytes (ESAT-6 and CFP-10 peptides)
TB2 antigen tube – includes additional peptides designed to stimulate CD8+ cytotoxic T lymphocytes

After incubation, IFN-γ concentration is measured using enzyme-linked immunoassay (ELISA).

Sensitivity and specificity

Because there is no gold standard for LTBI, assay performance is inferred from surrogate populations. Sensitivity is estimated in patients with microbiologically confirmed active TB and specificity is estimated in low-risk populations in low-incidence settings. In low-risk populations, current published data suggest sensitivity and specificity are 85–90% and >95%, respectively.

QFT-Plus is not affected by prior BCG vaccination, and when compared with tuberculin skin testing (TST), it has less cross-reactivity with most non-tuberculous mycobacteria.

Interpretation of results

According to the manufacturer:

Positive: If either TB1 or TB2, after subtracting Nil, is ≥0.35 IU/mL
Negative: No antigen response with adequate mitogen control
Indeterminate:
- High Nil value (excess background IFN-γ)
- Inadequate mitogen response (suggesting immunosuppression or technical failure)

What to make of low-positive results?

In low TB-prevalence settings, results between 0.35 and 0.99 IU/mL are associated with a higher likelihood of false positivity.

These results should be:

Interpreted in the context of clinical and epidemiological risk
Correlated with exposure risk
Not used in isolation to make treatment decisions

In these low-risk individuals, repeat testing or discussion with a specialist is advised.

Who to test?

The primary purpose of LTBI testing is to identify individuals who would benefit from preventive treatment; therefore, testing should be targeted.

Common indications include:

Close contacts of a patient with infectious pulmonary TB (after the appropriate window period)
Migrants, refugees and expatriates from countries with high TB prevalence
Healthcare workers and others with occupational TB exposure
Immunocompromised patients, including:
- HIV infection
- Planned or current treatment with TNF-α inhibitors or other biologic agents
- Solid organ or stem cell transplantation candidates
- Chronic renal failure, haematological malignancy or chemotherapy
Pre-immunosuppression screening, including corticosteroids ≥15 mg prednisolone daily for >1 month.

Can QuantiFERON be used in children?

In young children, immune immaturity significantly affects the performance of IGRAs. In those under 5 years of age, QFT-Plus is considered unreliable, and the tuberculin skin test remains the preferred method of testing. For children aged 5 years and older, QFT-Plus may be used, particularly in those who have received previous BCG vaccination.

False-negative results are more common in younger children, and because the risk of progression to severe tuberculosis is higher in this group, a negative IGRA does not exclude infection.

Use in immunocompromised populations

Immunocompromised patients are at increased risk of LTBI progressing to active disease. Risk varies according to the patient’s immunosuppressive state, including underlying conditions (such as HIV infection, malignancy, transplant or renal failure requiring dialysis) and the type and intensity of immunosuppressive therapies used.

It is important to remember that QFT-Plus may produce:

False negative results, due to impaired T-cell function
Indeterminate results, reflected by a failed mitogen control

In these groups:

A negative result does not exclude LTBI
Repeat testing or alternative strategies (e.g. TST, specialist review) may be required
Clinical risk assessment is critical

Indications for treatment of latent TB

The decision to treat LTBI must be carefully assessed and depends on the following:

Pre-test probability
- Recent exposure
- Origin from or prolonged residence in high-burden countries
- Occupational exposure
Risk of progression to active disease
- Approximately 50% of lifetime risk occurs within 2 years of infection
- Highest-risk groups include:
     - Infants and young children
     - HIV infection
     - TNF-α inhibitor therapy
     - Transplant recipients
     - Chronic kidney disease, silicosis, haematological malignancy
    - Untreated or inadequately treated prior TB
Risks of treatment
- Hepatotoxicity (risk increases with age, alcohol use and pre-existing liver disease)
- Peripheral neuropathy
- Drug interactions and adherence challenges

How to Order QuantiFERON-TB Gold Plus Testing

Request Form Instructions:

Complete the Clinical Labs General Pathology Request Form, requesting Quantiferon Gold (TB). For referrers in WA, please use the WA Request Form.
Blood samples can be collected at any Clinical Labs collection centre.

Clinical Notes:

Provide the reason for testing (e.g., exposure to active case, immunosuppressed, dialysis etc.).

References

Denholm JT, Eisen DP, Fox G, McBryde ES, Street A. Management of Tuberculosis: A Guide for Clinicians. Greenhill Publishing, 2017. ISBN 978 0 648 13797 9.
Position statement on interferon gamma release assays for the detection of latent tuberculosis infection. Communicable Diseases Intelligence. 2017;41(4). Available from: https://www1.health.gov.au/internet/main/publishing.nsf/Content/cdi4104-c
Coulter C. Tuberculosis testing. Australian Family Physician. 2012;41(7).

About the Author

Dr Linda Dreyer

MBChB MMED (Path) – South Africa FRCPA

Dr Linda Dreyer

MBChB MMED (Path) – South Africa FRCPA

Location: Clayton (VIC)

Speciality: Infection Control Microbiology

Areas of Interest:

antimicrobials
infection control
molecular diagnostic assays in contemporary clinical microbiology

About

Dr Linda Dreyer completed her undergraduate studies in 1996, receiving a Bachelor’s degree in Medicine and Surgery (MBChB) from the Faculty of Health Sciences, University of Pretoria, South Africa.

Following four years of clinical practice, as Medical Officer in the Department of Family Medicine she commenced specialisation in 2000. She was appointed as Registrar in Clinical Virology at the University of Pretoria/ Gauteng Province where she worked for two years and in 2003 she was appointed as Senior Registrar in Microbiology.

Dr Dreyer received her Master’s degree in Clinical Microbiology (MMed (Path)) from the University of Pretoria in 2006. She worked as a consultant for the National Health Laboratory Services (NHLS) in Pretoria until January 2008. During her time at NHLS she was involved in teaching medical students and microbiology registrars and gave lectures to nursing staff, medical students and specialists. She also sat on the Infection Control Committee and the Antimicrobial Stewardship Committee of the Pretoria Academic Hospital.

She came to Melbourne and joined Australian Clinical Labs (formerly Healthscope Pathology) in 2008 as a Senior Registrar and obtained Fellowship of The Royal College of Pathologists of Australasia (FRCPA) in 2010.

Dr Dreyer has special interests in the appropriate use of antimicrobials, infection control and molecular diagnostic assays in contemporary clinical microbiology.

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