May I have your opinion on the use of IGRA , especially T-spot on LTBI? I have personally experience the lack of reproducibility of T-spot, a first test being borderline positive and the repeated one negative, with only one spot. When a test is so inconsistent, even if it being very specific and sensitive, can we still use it in confidence to diagnose LTBI?
Hypothetically,
a) If a HCW, healthy with low risk of TB infection has a first positive test for healthy employment screening, will you treat them for LTBI once active TB is excluded? or will you repeat the test? what if it comes back borderline, or negative?
b) And if a patient, healthy with low risk of TB infection has a borderline test, does a second negative or positive test mean anything much?
What are your usual practise? Thank you in advance for the opinions.
First, we need to check the negative control and positive control. A valid result would be expected to have few or no spots in the Negative Control and 20 or more spots in the Positive Control. A Negative Control spot count more than 10 spots should be considered as Indeterminate. Where the Positive Control spot count is < 20 spots, it should be considered as “Indeterminate”, unless either Panel A or Panel B is Positive.
Panel A or Panel B spot count is such that the (Panel minus Negative Control) spot count is 5, 6 or 7 spots should be considered Borderline (equivocal) and re-testing by collecting another fresh patient specimen is recommended. Based on my experience, the inconsistency may due to the wrong storage temperature (most of the people store at cold and transport in cold), use of inappropriate blood collection tubes and deliver to the lab more than 8 hours after blood collection. Normally, another patient’s specimen is collected and repeat for the test if Borderline or Indeterminate result appeared.
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