What are the QuantiFERON cutoffs?

A result is positive when TB antigen minus nil is at least 0.35 IU/mL and at least 25% of the nil value, regardless of the mitogen. It is negative when antigen minus nil is below that threshold and the mitogen minus nil is at least 0.5 IU/mL. If the mitogen response is too low or the nil is too high, the result is indeterminate.

Why does a high nil value cause an indeterminate result?

The nil tube measures background interferon-gamma without stimulation. If the nil exceeds 8.0 IU/mL the background is so high that a true antigen signal cannot be distinguished, so the run is reported as indeterminate and should be repeated.

What does an indeterminate mitogen mean?

The mitogen tube is a positive control that should always trigger a strong immune response. If mitogen minus nil is below 0.5 IU/mL in an otherwise negative pattern, the patient's cells failed to respond even to a universal stimulus, so a true negative cannot be confirmed and the test is indeterminate.

What is the borderline zone?

Values just above the 0.35 IU/mL cutoff, conventionally between 0.20 and 0.70 IU/mL, are less reproducible and a repeat test may flip the result. This tool flags antigen-minus-nil values in that band so you can consider confirmatory retesting even though the formal call is positive or negative.

Does this replace the laboratory report?

No. It reproduces the published QIAGEN decision logic for education and cross-checking. The validated software in the analyser and the issuing laboratory remain the authoritative source, and clinical interpretation must consider TB risk, symptoms, and prior results.

QuantiFERON-TB Gold Interpretation Calculator

QuantiFERON-TB Gold Plus is an interferon-gamma release assay (IGRA) for tuberculosis infection. It measures the immune response to TB-specific antigens against background and positive controls, and a fixed decision algorithm turns those numbers into a positive, negative, or indeterminate call.

How it works

The classification depends on two corrected values:

TB-Nil      = TB antigen IFN-γ - Nil IFN-γ
Mitogen-Nil = Mitogen IFN-γ - Nil IFN-γ

The QIAGEN algorithm is then applied in order:

Nil > 8.0                                   -> Indeterminate
TB-Nil >= 0.35 and TB-Nil >= 25% of Nil     -> Positive
else if Mitogen-Nil >= 0.5                   -> Negative
else                                         -> Indeterminate

The Positive test fires regardless of the mitogen, because a clear antigen response is meaningful on its own. The Negative test requires a competent mitogen control, otherwise the result is indeterminate.

Interpretation and notes

A positive IGRA indicates immune sensitisation to TB antigens but does not distinguish latent infection from active disease; clinical assessment and imaging decide that. An indeterminate result usually reflects a high background nil or a failed mitogen control and should be repeated. Values just above the 0.35 IU/mL cutoff fall in a less reproducible borderline zone (around 0.20 to 0.70 IU/mL) where a repeat may change the call, which this tool flags. The calculator reproduces the published decision logic only; the issuing laboratory’s validated software remains authoritative.