The cycle of threshold value of cartridge based nucleic acid amplification test and its correlation with time to positivity in AFB culture

Background: The interpretation of the result of CBNAAT is done on the basis of CT value as high, medium, low and very low. CT value may correlate with the culture growth andthe TTP, so that treatment can be started as soon as possible. Aims and objectives: To correlate the relationship between CT value and its TTP on culture growth. Methods: This was a prospective study conducted from July 2014 to March 2016. The samples collected consisted of bronchial samples, pleural samples and others (CSF, lymph node biopsies, tissue samples) of suspected cases of Tuberculosis. The samples were sent for direct smear and culture as well as CBNAAT. The data was analyzed and a comparison was drawn among positive detected patients of CBNAAT with respect to CT value and TTP of culture growth. Result: Among 1000 samples CBNAAT was detected positive in 162 samples (bronchial samples were 61/383, sputum were 37/130, pleural samples were 9/221 and others were 55/266). There is significant association between CT values and growth at weeks. (Chi Square value = 11.827; p-value = 0.008). The samples with high and medium detected CT values have shown earlier culture positivity. Conclusion: On the basis of CT-Value (high, medium, low and very low), TTP on growth in culture can be predicted.