Public Health and Environmental Laboratories

Harvest Order Choices: QFT

Synonyms: Interferon Gamma Release Assay for tuberculosis

Program: Automated Assays

Unit: QFT

Useful For: Detection of M. tuberculosis infection

Method: Interferon- Gamma detection and quantification by ELISA in response to TB antigens

Charge: None

Request Form: LAB-10

Container/ Tube: TB antigen 1 ml tube (red top), Nil control 1 ml tube (grey top), Mitogen control 1 ml tube (purple top)

Type: Blood

Volume: 1 ml per tube

Collection Instructions: Collect 1 ml (marked by line on the tube) by venipuncture directly into each of the QuantiFERON- TB Collection Tubes, tubes should be at 17-25 C at time of blood filling. Write patient’s name and patient’s identification number on the labels provided one strip of five labels is provided for each patient. Note that one label is designated for GRAY, RED and PER top tubes, one for the FORM and one extra. Apply labels vertically on the tubes, so the barcode appears straight up and down. The bar code will be read by machine in the laboratory so this step is important. Do not cover the black fill line or the colored stripe on the manufacturer’s label with your label. Match the color cap with the color name pre-printed on the label. Immediately after filling tubes, shake them ten times just firmly enough to ensure the entire inner surface of the tube is coated with blood to solubilize the antigen on tube walls. Fill in date, time and your initials on QFT test requisition form.

Storage/ incubation: Place tubes upright at room temperature in the blood tube rack until they are incubated. DO NOT refrigerate prior to incubation. Within 16 hours of collection tubes must be incubated at 37C.

Causes for Rejection: All three tubes are not received, missing Request for QuantiFERON-TB Gold form, Any unlabeled specimens, Specimens without necessary quantity of blood (determined by a marker on the tube), missing collection vs. incubation times or collection vs. incubation times that fall outside the established parameters.

Interpretation: For a test to be considered positive, the TB antigen tube value minus the Nil tube value must be ≥0.35 IU/mL and ≥25% of Nil value (FDA cleared breakpoint).

Reference Interval: Negative

Limitations: Results from QFT testing must be used in conjunction with each individual’s epidemiological history, current medical status, and results of other diagnostic evaluations

 Individuals with Nil values greater than 8 IU/ ml are classed as “Indeterminate” because a 25% higher response to TB Antigens may be outside the assay measurement range

The predictive value of a positive QFT result in diagnosing M. tuberculosis infection depends on the probability of infection, which is assessed by historical, epidemiological, diagnostic, and other findings.

A diagnosis of LTBI requires that tuberculosis disease must be excluded by medical evaluation including an assessment of current medical and diagnostic tests for disease as indicated.
A negative result must be considered with the individual’s medical and historical data relevant to the probability of M. tuberculosis infection and potential risk of progression to tuberculosis disease, particularly for individuals with impaired immune function. Negative predictive values are likely to be low for persons suspected to have M. tuberculosis disease and should not be relied on to exclude disease.

The ’standard’ and ‘high altitude’ blood collection tubes have both been calibrated for use at altitudes as specified in Section 5. Using either type of tube outside the recommended altitude ranges may result in an incorrect volume of blood draw and lead to an incorrect diagnostic outcome. If usingthe tubes outside the specified ranges of altitude, or if tubes do not fill to the indicator line, it is recommended to draw blood using a syringe and transfer blood to the QFT tubes manually.

Unreliable or indeterminate results may occur due to:

• Deviations from the procedure described in the package insert
• Incorrect transport/ handling of blood specimen
• Excessive levels of circulating IFN-γ or presence of heterophile antibodies
• Longer than 16 hours from blood specimen drawing to incubation at 37 C ± 1C

Availability: N/A