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"The right test, on the right specimen, collected at the right time, is essential for reliable diagnosis" Dr Prashant Goyal

Beyond Rifampicin: The Clinical Importance of Detecting Isoniazid Resistance

 

Rapid molecular diagnostics have transformed the diagnosis and management of tuberculosis (TB). CBNAAT (Cartridge-Based Nucleic Acid Amplification Test) and MTB PCR enable early detection of Mycobacterium tuberculosis (MTB) and provide rapid information on drug resistance. Traditionally, most attention has focused on detecting Rifampicin (RIF) resistance, as it is considered an important marker for multidrug-resistant tuberculosis (MDR-TB). However, limiting resistance detection to rifampicin alone may overlook another clinically significant challenge - Isoniazid (INH) resistance.

Isoniazid is one of the most effective first-line anti-tubercular drugs and plays a major role in early bacterial killing during treatment. Resistance to INH can occur independently (INH monoresistance) or together with rifampicin resistance. When INH resistance remains undetected, patients may continue receiving standard treatment regimens that are less effective for their infection.

The clinical consequences of missed INH resistance can be significant. Patients may experience slower response to therapy, prolonged infectivity, treatment failure, or relapse after completion of treatment. Continued exposure of resistant organisms to ineffective drugs may also increase the risk of developing additional resistance over time.

Historically, rifampicin resistance received greater emphasis because it is strongly associated with MDR-TB and is easier to use as a rapid treatment decision marker. However, relying solely on rifampicin resistance assumes that all clinically important resistance patterns will be identified - which is not always true. Patients with RIF-sensitive but INH-resistant TB may not be recognized early and therefore may not receive optimized therapy.

This is where modern molecular diagnostics offer an important advantage. Advanced MTB PCR platforms that detect both RIF and INH resistance provide a more complete drug resistance profile at the initial stage of diagnosis. INH resistance is commonly associated with mutations in the katG gene and inhA promoter region, and identifying these mutations can provide valuable information for treatment planning.

Simultaneous detection of INH and RIF resistance supports clinicians in selecting more appropriate treatment regimens earlier in the disease course. It may reduce diagnostic delay, improve treatment outcomes, and help prevent progression toward more complex drug-resistant TB.

For laboratories, adding INH resistance detection increases the clinical value of molecular TB testing. For clinicians, it enables more informed decision-making. For public health systems, early identification of resistance patterns contributes to better TB control and may help reduce transmission of resistant strains.

The future of TB diagnostics is moving beyond simply confirming the presence of Mycobacterium tuberculosis. The focus is increasingly shifting toward early and comprehensive resistance detection.

Detecting rifampicin resistance remains essential—but detecting Isoniazid resistance alongside rifampicin resistance provides a clearer picture, supports personalized treatment, and ultimately improves patient care.

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