TB Manual

Pre-analytical errors. For the TST, this includes possible issues with the PPD solution and administration (e.g., volume injected), while for IGRA these may include improper shaking of the tube after collection, delays in transport and/or incubation of samples, or errors in the actual time the sample is incubated.

Within-person variations over time. For the TST, responses may vary by up 5 mm in reaction size between tests. For the IGRA, a previous systematic review suggested there was a mean difference in two consecutive samples of 0.2 IU/mL between two QFT tests; similar data for T-SPOT.TB was not reported.

Analytical errors. For the TST, a mis-recording of TST measurements may lead to an incorrect conclusion of a change in induration on a second test. For the IGRA, if not recorded automatically, data-entry errors on quantitative values and interpretation may occur.

Test-retest variation. While agreement is generally quite good when the same sample is tested in the same lab twice, there may be variation in the spots counted or IU/ml measured from the same sample.

Inter-laboratory variation. There may be different measurements when the same sample is tested in different labs.

TST-induced variation. A previous TST may “boost” a subsequent IGRA result. Conversions in a systematic review were uncommon but did occur with both T-SPOT.TB and QFT. Whether the risk of disease among persons with a boosted response is different than risk among those without a boosted response is unclear.