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Test Code QFTTB Mycobacterium tuberculosis by QuantiFERON-TB Gold Plus, Blood

Performing Laboratory

CCHS-Christiana Care Health Services

Methodology

Enzyme-Linked Immunosorbent Assay (ELISA)

Specimen Requirements

Container/Tube:  Nil Control (grey cap), TB-Antigen 1 (green cap), TB-Antigen 2 (yellow cap), Mitogen (purple cap). 

Standard Altitude:  QuantiFERON-QTB Gold In-Tube Collection Kit (Supply T628)

Collection Instructions:  1.  Special collection procedure must be followed.

2.  Collect 1 mL of blood by venipuncture directly in each of the QuantiFERON-TB Gold IT tubes.  As 1 mL tubes draw blood slowly, keep the tube on the needle for 2-3 seconds once the tube appears to have completed filling, to ensure that the correct volume is drawn.  The black mark on the side of the tubes indicates the 1 mL fill volume.  Immediately after filling the tubes, shake them ten (10) times just firmly enough to ensure the entire surface of the tube is coated with blood.  Transport to the lab within 4-8 hours of collection at ambient temperature

    

Reference Values

TB Ag - Nil

The TB antigen tube minus the Nil tube (TB Ag-Nil) is considered positive when it is > 0.35 IU/mL.

 

Nil

The Nil tube value is used to determine if the patient has a preexisting immune response which could cause a false positive reading.  Inorder for the test to be valid, the Nil tube must have a value of < 8.0 IU/mL.

 

Mitogen - Nil

The mitogen control (Mitogen-Nil value) is used to assure that the patient has a healthy immune response and also serves as a control for correct blood handling and incubation.  It is used to detect false negative readings.  The Mitogen-Nil value must be > 0.5 IU/mL.

 

Positive

Interferon-gamma response to M. tuberculosis antigens detected suggesting M. tuberculosis infection.  Positive results in patients at low-risk for TB should be interpreted with caution and repeat testing on a new sample is recommended.  False positive results may occur in patients with M. marinum, M. szulgai and M. kansasii.

 

Negative

No interferon-gamma response to M. tuberculosis antigens detected.  M. tuberculosis infection NOT likely.

 

Indeterminate

An indeterminate result may occur due to a low lymphocyte count, reduced lymphocyte activity or inability to generate interferon-gamma.  If clinically indicated, recollecte specimen after 8 weeks.

Useful For

Quantiferon-TB Gold Plus is an indirect test for Myobacterium tuberculosis complex infection.  The test detects both latent tuberculosis and active disease but can not distinguish between them.  May be used in place of the Tuberculin Skin Test (TST).  Preferred for testing persons who have received BCG.

Specimen Type

Whole blood, tubes specific to this test are required

Specimen Minimum Volume

1 mL per tube (4 tubes)

Day(s) Performed

3-4 times per week