TABLE 2.
Interferon-gamma-release assays
| The QuantiFERON-TB Gold In-Tube assay (Cellestis Inc, Australia) | |
| What it measures | Whole blood ELISA, which measures the interferon-gamma concentration in blood after incubation with the tuberculosis-specific antigens ESAT-6, CFP-10 and TB7.7. Response to both mitogen stimulation and a control are also measured to ensure quality of the assay |
| Specimen and processing requirements | Requires the collection of 3 mL of blood, 1 mL per tube (antigen, mitogen, nil). Samples must be processed within 12 h of collection. These tubes are incubated for 16 h to 24 h, after which time the amount of interferon-gamma produced is measured via ELISA |
| Interpretation of results | Results are reported as either positive (measured interferon-gamma 0.35 IU/mL or greater, and greater than 50% above the control), negative (measured interferon-gamma 0.35 IU/mL or less) or indeterminate (measured interferon-gamma less than 0.35 IU/mL and suboptimal mitogen response of less than 0.5 IU/mL, or measured interferon-gamma of 0.7 IU/mL or less above the control/nil response) |
| The T.SPOT–TB test (Oxford Immunotec, United Kingdom) | |
| What it measures | Ex-vivo enzyme-linked immunoassay, which estimates the number of T cells producing interferon-gamma after exposure to Mycobacterium tuberculosis-specific antigens. Isolated peripheral blood mononuclear cells from the patient are placed into microlitre wells, panel A (ESAT-6), panel B (CFP-10), nil control or mitogen, and then incubated to allow stimulation of any sensitized T cells present. The number of T cells producing interferon-gamma are then counted and quantified |
| Specimen and processing requirements | The T.SPOT–TB assay requires 8 mL of blood for children older than 10 years of age, 4 mL for children between two and 10 years of age, and 2 mL for children younger than two years of age (with double the volume requirement for immunocompromised patients). Samples must be processed on the same day (within 8 h of collection) |
| Interpretation of results | Results are presented as the number of spot-forming T cells (SFCs), and classified as either positive, negative, borderline or indeterminate. A result is defined as positive when either panel A or B contains eight or more SPCs more than the negative control, negative when both panel A or B contains four or less SPCs than the negative control, and borderline, when the highest of panel A or B contains five, six, or seven SPCs more than the negative control. A borderline test is considered equivocal and should be repeated |
| The presence of a satisfactory reaction (greater than 20 SFCs) to the mitogen-positive control demonstrates T cell function and validates the assay result. An indeterminate result is reported when high background levels prevent interpretation (greater than 10 SFCs in nil/control wells), or when less than 20 SFC are detected in the mitogen-positive control wells | |