Diagnostics of tuberculosis

If tuberculosis is suspected, a combination of anamnesis, laboratory and X-ray diagnostics are needed for a differential diagnosis. Using a radiograph of the thorax, pulmonary tuberculosis can be recognised and therapeutic progress can be assessed.

The tuberculin skin test, also known as PPD test, for detection of latent tuberculosis is only rarely used nowadays due to possible cross reactivity with nontuberculous mycobacteria (NTM) and previous BCG vaccination. Today so-called interferon γ release assays (IGRAs) are used, where interferon γ-producing T cells are detected after stimulation with M. tuberculosis-specific antigens. Compared to PPD, IGRAs are more specific and are not compromised by BCG vaccination. 

Microbiological pathogen detection is usually performed using pulmonary samples like sputum but also gastric juice, urine, liquor, puncture and tissue samples can be used.

For a quick clarification about whether or not a patient is infectious, microscopy should be used. Acid-fast bacilli like mycobacteria can be detected by Ziehl-Neelsen or auramin staining using light or fluorescence microscopy. Microscopic analysis is a quick method but it is impossible to distinguish living bacteria from dead or to discriminate between different species. Furthermore, at least 103-104 bacteria/ml are required for a positive result.

Mycobacteria are quite slow-growing. Therefore, rapid-growing accompanying bacterial flora has to be eliminated by decontamination for culture detection. Afterwards, cultures on solid and in liquid media are inoculated. The limit of detection of culture is roughly 100 bacteria/ml. Positive results in liquid media are available after one to two weeks, on solid media after three to four weeks. However, negative results take six (liquid media) to eight weeks (solid media).

Nucleic acid amplification assays (NAT) for direct pathogen detection in culture or patient samples are important to supplement culture and microscopy. NAT are sensitive and specific and results are available after several hours. Modern diagnostics do not only benefit from NAT in case of microscopically negative samples but also in microscopically positive and doubtful samples, samples from HIV patients or children or if MDR- or XDR-TB is suspected. Nevertheless, DNA detection cannot be used for therapy control as dead bacteria are detected as well.

Also differentiation of the species belonging to the M. tuberculosis complex and of NTM is usually performed using molecular biology.

Conventional drug susceptibility testing in liquid or on solid media usually takes an additional one to four weeks. In contrast, molecular resistance testing, which is recognised worldwide, provides a vast time benefit. That is the only way therapy can quickly be adjusted and selection of resistant pathogens can be avoided. Therefore, detection of the most common mutations that mediate resistance to rifampicin and isoniazid allows an early identification of multidrug-resistant tuberculosis.

Furthermore, identification of certain mutations that are associated with the additional resistance to second-line drugs, offers valuable clues to the presence of a case of extensively drug-resistant tuberculosis.