Culture positivity

By Ranjan Perera | 20 Nov, 2016

under properly controlled screening TB suspects, what are the approximate culture positivity rate, microscopy positivity rate and GeneXpert positivity rate?



Mark Harrington Replied at 4:11 AM, 20 Nov 2016

The framing of the question makes it impossible to answer accurately.

What is the correct relation of an unmeasured numerator to an unknown denominator?

Masoud Dara, MD Moderator Replied at 5:22 AM, 20 Nov 2016

Dear Ranjan,

Building on Mark's point, would you please elaborate further your question? Do you mean among presumptive pulmonary TB individuals, which percentage is expected to to have positive results of sputum smear examination, mycocateriologic culture and Genexpert?

Also as reminder to all us, we avoid using the term "TB suspect", which is replaced by presumptive TB. This to ensure people's dignity.

Thank you and all the best,

Praharshinie Rupasinghe Replied at 5:32 AM, 20 Nov 2016

Dear colleagues,

Our laboratory perform ZN microscopy, Genexpert and LJ culture.

Among presumptive TB cases roughly how much is expected to be smear
positive, GeneXpert positive and Culture positive if our clinicians screen
the patients according to the screening guidelines.


Tafara Zanamwe Replied at 1:21 AM, 21 Nov 2016

Dear Colleagues
I think that the positivity rate depends upon epidemiology of TB in the catchment area of the lab, the strategy for identifying presumptive TB patients (e.g. intensified case finding in high HIV prevalence settings may tend to increase the number of presumptive patients for whom bacteriological confirmatory test is negative) , and the performance of the lab. For a lab performing well in quality assurance my approach would be to compare the lab's positivity rates for each test with its positivity rates from the recent past (e.g. Q1 2016 vs Q1 2015). This will give some way to detect deviations. However even though I should still mention that when a TB program is working well then after some time one expects that the positivity rates will go down and that should be expected as well. In theory, in the beginning the diagnosed patients include incident cases and duration of disease case not previously placed on treatment. With time for a program effectively combining case finding and treatment, then the cases diagnosed later are composed of only incident cases.