To Editor,
The global health burden of tuberculosis (TB) is still high. TB was the all-time highest morbidity in terms of infectious disease before the COVID-19 pandemic.[1] The World Health Organization (WHO) and the Indian government have taken the initiative for a TB control and elimination program. Various programs on TB control and elimination are carried out by the Indian government including the National Tuberculosis Control Program (NTCP), the Revised National Tuberculosis Control Program (RNTCP), and now the National Tuberculosis Elimination Program (NTEP).[2]
Previously, TB diagnosis was very cumbersome, and laborious due to Ziehl-Nielsen (Z-N) staining and culture. After the approval of cartridge-based nucleic acid amplification techniques (CBNAAT) for TB diagnosis in 2010, the scenario completely changed. RNTCP centers are using CBNAAT for fast TB diagnosis. Previously, TB treatment was started after a culture-positive result, but due to CBNAAT, prompt diagnosis and early detection became possible.[3]
A good prospective and comparative study between acid-fast stain (Z-N stain) and CBNAAT was conducted at the center of Meerut, where they found good results in CBNAAT.[4] These outcomes we know very well. However, there are various limitations they must notice and discuss in the manuscript. Whenever we are going to compare two methods of diagnosis, we should have to do a gold standard test simultaneously. Culture is the gold standard and the author did not write anything about it. To detect TB in specimens, the test has different sensitivity. Z-N stain has lesser sensitivity around 40–70% as per various studies because it’s required a high bacillary load, while CBNAAT has 90–95% sensitivity due to its low bacilli count (16 base pair (bp) for CBNAAT ultra and 131 bp for CBNAAT). This is very helpful in detecting TB in low–bacillary-load pulmonary as well as extrapulmonary specimens.[5]
It detects the genetic material of TB bacilli; unfortunately, it is not able to detect acute, chronic, active, or past infections. Starting treatment on the basis of CBNAAT positive sometimes becomes hazardous. We can say that the CBNAAT negative result confirms that the patient specimen doesn’t have TB bacilli, but we can’t say that the CBNAAT positive means result the patient has a current TB infection. It may be due to a past infection, dead bacilli, or under treatment. To diagnose the active TB infection, culture is the ideal test. Due to this limitation, now RNTCP centers send the sample to State Research Laboratory (SRL) for culture and Drug Susceptibility Testing (DST).[6]
Culture and DST results are helpful in selecting the treatment regimen. Not improving TB patient’s regimen change is dependent on DST but current and active TB infection should be confirmed with solid/liquid culture methods. Due to this limitation, CBNAAT cannot be used as a gold standard test for TB diagnosis. It cannot take on the complete burden of TB diagnosis. Due to the unavailability of culture processes or results, we cannot compare both methods with different sensitivity.
In a nutshell, we want to say that whenever any comparative analysis or study is conducted, at least add one gold standard test to find out the efficacy of the diagnostic test. Without adding a gold standard test, the comparative analysis study becomes incomplete.
Response to.
Comparative analysis of CBNAAT (GeneXpert) and Ziehl-Nielsen staining test as diagnostic modalities of tuberculosis.
Financial support and sponsorship
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Conflicts of interest
There are no conflicts of interest.
References
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