From the Guidelines
The confirmatory test for active tuberculosis is a positive culture of Mycobacterium tuberculosis from a clinical specimen. This is based on the most recent and highest quality evidence available, which emphasizes the importance of culture in confirming active TB disease 1. While other tests such as nucleic acid amplification (NAA) and microscopic examination using acid-fast bacilli (AFB) smears can provide rapid preliminary results, they are not a replacement for culture.
Key Points to Consider
- Culture of sputum or other clinical specimen provides a definitive diagnosis of TB disease, with results available within 28 days using recommended rapid methods 1.
- NAA tests can facilitate rapid detection and identification of M. tuberculosis, but should not replace culture and drug-susceptibility testing in patients with suspected TB disease 1.
- A negative culture result is obtained in approximately 14% of patients with confirmed pulmonary TB disease, highlighting the importance of considering clinical context and other diagnostic findings 1.
- The use of liquid culture and DNA probes can provide faster results and improve the accuracy of diagnosis 1.
- Other tests, such as histopathological examination showing caseating granulomas, can support diagnosis in tissue samples, but are not a substitute for culture. Some key considerations for clinicians include:
- Obtaining specimens for bacteriologic confirmation and susceptibility testing from patients with TB or suspected of having TB 1.
- Using standard methods for collection, transportation, concentration, processing, and culturing of specimens to ensure accurate results 1.
- Considering the use of new techniques, such as direct amplification tests and mycolic acid analysis by high-performance liquid chromatography (HPLC), to enhance and hasten the direct identification of bacilli in clinical specimens and cultures 1.
From the Research
Active TB Confirmatory Test
The confirmatory test for active tuberculosis (TB) is a crucial aspect of diagnosing and managing the disease. Several studies have investigated the diagnostic efficacy of various tests, including the Xpert MTB/RIF assay and MTB culture.
- Xpert MTB/RIF Assay: This molecular diagnostic tool can quickly detect Mycobacterium tuberculosis (MTB) and rifampicin resistance, with high sensitivity and specificity 2, 3, 4.
- MTB Culture: Considered the gold standard for diagnosing TB, MTB culture is a time-consuming process that can take several weeks to yield results 2, 3, 5.
- GeneXpert MTB/RIF Assay: This assay has been shown to be a rapid and accurate tool for detecting MTB in smear-negative sputum specimens, with advantages over ZN smear microscopy and MTB culture 3, 6.
Diagnostic Efficacy
The diagnostic efficacy of these tests has been evaluated in various studies, with the following results:
- Sensitivity and Specificity: The Xpert MTB/RIF assay has been shown to have high sensitivity (86-98%) and specificity (88-99%) for detecting TB and rifampicin resistance 2, 3, 4.
- Positive and Negative Predictive Values: The positive predictive value of the Xpert MTB/RIF assay has been reported to be 71-95%, while the negative predictive value has been reported to be 95-99% 2, 3.
Comparison of Tests
The Xpert MTB/RIF assay has been compared to other diagnostic tests, including smear microscopy and MTB culture, with the following results:
- Comparison with Smear Microscopy: The Xpert MTB/RIF assay has been shown to be more sensitive than smear microscopy for detecting TB, especially in smear-negative patients 4.
- Comparison with MTB Culture: The Xpert MTB/RIF assay has been shown to be faster and more sensitive than MTB culture for detecting TB, although culture remains the gold standard for diagnosing TB 2, 3, 5.