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Paediatric Laboratory Medicine

Quantiferon TB Gold

Background

  • Quantiferon TB Gold is an in vitro diagnostic test for cell mediated immune response to peptide antigens that simulate Mycobacterial proteins.
  • Individuals infected with M.tuberculosis (TB) usually have lymphocytes in their blood that recognize these antigens and secrete IFN-gamma that is subsequently detected by ELISA.
  • QFT is intended for use in conjunction with risk assessment, radiography and other medical and diagnostic evaluations.
  • A negative test does not rule out TB.

Clinical Significance

Quantiferon TB Gold is an indirect test for detecting latent tuberculosis infection (LTBI) and a helpful aid for diagnosing Mycobacterium tuberculosis complex infection in sick patients.

Test Name

Quantiferon TB Gold

Alternate Name/ Synonym

QFT

Test Code

QUANTB

Division

Molecular Microbiology - Bacterial

Method

ELISA

External Proficiency Testing

CAP

Turn Around Time

Results within 2 weeks of receipt of specimens (dictated by demand)

Disease/Condition

Quantiferon TB Gold is an indirect test for detecting latent tuberculosis infection (LTBI) and tuberculosis diseases.

Specimen Type

Whole blood

SPECIALIZED TUBES (QFT TB Kit) REQUIRED FOR TESTING - Kit contents (3 tubes):

  • Nil Control (Grey Cap with white ring)
  • TB Antigen (Red Cap with white ring)
  • Mitogen Control (Purple Cap with white ring)

Minimum Specimen Requirements

Collect 1mL of blood into each QFT blood collection tube (NOTE: Fill to black line)

  • Immediately after filling tubes invert tubes ten (10) times just firmly enough to ensure the entire surface of the tube is coated with blood.

Tubes should be between 17–25°C at time of blood filling.
Over-energetic shaking may cause gel disruption and could lead to aberrant results.

Optimal Collection Time

QFT TB Kit 

NOTE: Specimens that DO NOT MEET the transport requirements will be REJECTED. See instructions below.

1. Specimens Delivered and Received within 12 Hours of Collection

  • Tubes MUST BE received in the Microbiology Laboratory within 12 hours of collection.
  • DO NOT refrigerate or freeze samples, ship at room temperature.

OR

2. Specimens that CANNOT meet the 12 hour transport time (from time of Collection). INCUBATION AND CENTRIFUGATION OF TUBES BY CLIENT PRIOR TO SUBMISSION IS REQUIRED.

  • The referring lab should incubate specimen on site. Do not refrigerate or freeze specimen.
  • Within 16 hours of collection, incubate at 36°C–38°C for 16–24 hours. (Invert tubes 10x again prior to incubation.) Keep tubes UPRIGHT in incubator. Incubator does not require CO2 or humidification.
  • After incubation, centrifuge samples (15 min at 2000 to 3000 RCF (g)). Ship to Microbiology lab at 2–8°C. Indicate on requisition that specimens have been incubated and centrifuged.

Storage/Transportation

Please see: Optimal Collection Time above.

Special Requirements

Read: Blood Collection and Tube Handling Technique - Quantiferon (QFT) TB Gold (on page 3)

Specialized specimen collection tubes required for testing – see Specimen Type / Transportation

Reference Range

Positive Test Result

> 0.350 IU/mL

Negative Test Result

< 0.350 IU/mL

Indeterminate Result

Due to high Nil value or low mitogen response

Interpretation

For assistance with the interpretation please contact the Infectious Diseases / TB Program through The Hospital for Sick Children location 416-813-7500.