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Interferon Gamma Release assay - Provincial

Discipline

Microbiology

Overview

Description

Interferon Gamma Release assay is an important test for the diagnosis and confirmation of latent Mycobacterium tuberculosis infection (LTBI). In immigrant populations coming from countries where BCG vaccination is administered, tuberculin transdermal test is not reliable in diagnosis of LTBI. In such populations, IGRA is the ideal option. IGRA is not usually indicated for diagnosis of active infection.

Ordering recommendations

  • When the risk of infection, of progression to disease and of a poor outcome are high
  • When false-negative or false-positive Tuberculosis Skin Test (TST) results are suspected
  • People who have received BCG as a vaccine after infancy (1 year of age) and/or have received BCG vaccination more than once.
  • People from groups that historically have poor rates of return for TST reading.
  • In children with suspected TB disease, IGRAs may be used as a supplementary diagnostic aid in combination with the TST and other investigations to help support a diagnosis of TB. However, IGRA should not be a substitute, or obviate the need, for appropriate specimen collection. A negative IGRA does NOT rule out active TB at any age and especially not in young children.
  • Repeating an IGRA might be useful when the initial TST result is indeterminate, borderline or invalid and a reason for testing persists.

Specimen Information

Specimen types accepted

  • Whole blood

Specimen collection container

  • Special QuantiFERON TB Gold Plus tubes: 1 GRAY, 1 RED, and 1 LAVENDER and 1 YELLOW

Collection procedure

  • QuantiFERON – TB Gold Plus Collection and Incubation Training Video
  • If using a butterfly needle – a “purge” tube should be used to ensure the tubing is filled before using the QuantiFERON TB Gold Plus tubes
  • Collect 1 mL of blood into each tube (Note: these tubes draw relatively slowly)
  • Draw order: Grey-top, Green-top, Yellow-top, Purple-top
  • Keep the tube on the needle for 2-3 seconds once the tube appears to be filled to ensure the correct volume is drawn
  • Ensure the blood is at the fill line (black mark) on the side of the tubes
  • Label tubes so that the black mark and the blood level are visible
  • Keep tubes upright after labeling
  • Immediately after filling the tubes (using a firm handshake motion), shake the tubes ten (10) times to ensure the inner surface of the tube is coated with blood to dissolve the antigens on the inner walls of the tube
  • See Stability/Storage

Required volume

  • 1 mL in each tube. Note: tubes must be filled to the fill line (black mark) on tube

Transport and stability

  • Incubate 37°C for 16-24 hours within 16 hours of collection. Centrifuge at 2000-3000 RCF for 15 minutes. If the gel plug does not separate the cells from the plasma, the tube should be recentrifuged.
  • Plasma samples can be stored in centrifuged QFT-Plus Blood Collection Tubes for up to 28 days at 2-8°C. Do NOT disturb or touch the gel separator.
  • After incubation and centrifugation, store tubes upright at 2-8°C. Ensure tubes remain upright during shipment.
  • Referred in specimens or if centrifugation is unavailable: After incubation of the blood collection tubes for 16-24 hours at 37°C mark the incubated date and time on tubes. Tubes may be held after incubation for up to 72h prior to centrifugation. Tubes need to arrive at Saskatoon within 3 days of incubation or they will be canceled.

Last Updated: May 6, 2024