Anyone with a cough that lasts for two weeks or more or with unexplained chronic fever and/or weight loss should be evaluated for TB11. Chest X-ray is the primary radiologic evaluation of suspected or proven pulmonary TB. Radiological presentation of TB may be variable but in many cases is quite characteristic. Radiology also provides essential information for management and follow-up of these patients and is extremely valuable for monitoring complications. Chest X-ray is useful but is not specific for diagnosing pulmonary TB, and can be normal even when the disease is present4,5. Therefore, it cannot provide a conclusive independent diagnosis and needs to be followed by sputum testing. Many of the so-called unusual manifestations of adult TB are actually usual manifestations of primary disease. Post-primary TB in adults typically manifests as a heterogeneous, often cavitary opacity in the apical and posterior segments of the upper lobes and the superior segments of the lower lobes. Lymphadenopathy is rare. Cavitation is the hallmark of post-primary TB and appears in about half of all patients. Patchy, poorly defined consolidation in the apical and posterior segments of the upper lobes and in the superior segment of the lower lobe is also commonly observed4,5,14,15. However, post-primary disease activity cannot be accurately assessed by chest radiography. Radiographic stability for 6 months and negative sputum cultures is the best indicator of inactive disease16. The descriptive terms 'inactive' or 'old' TB should be discarded in favor of radiographically stable TB, as viable bacilli may persist despite adequate therapy. Post-primary TB heals with parenchymal scarring and nodules. An important task for radiology is to determine whether these residual findings are indicative of active disease. For this, chest X-ray has limited value, since it can only establish that a lesion is stable, and stable lesions can contain active bacilli4,5.

Although chest X-ray is the primary diagnostic tool for evaluating pulmonary TB, chest computed tomography (CT) is generally required to detect fine lesions that can be overlooked on chest X-ray, to define equivocal lesions, or to evaluate complications4,14. Chest CT is an effective diagnostic method when plain films are normal or inconclusive, and it provides valuable information for managing the illness. Chest CT can add valuable information for detecting bacterial activity. Branching opacities, cavitation, or consolidation are clear signs of active TB, but active disease must be confirmed by analyzing sputum for the presence of bacilli. A significant radiologic finding in chest CT is the "tree-in-bud" pattern, consisting of multiple branching linear structures that represent bronchogenic dissemination of disease with caseating necrosis in the respiratory and terminal bronchioles14. These branching opacities have a lobar or segmental distribution and are considered reliable markers of activity. Tree-in-bud opacities are also seen in other infections, but when visualized in combination with cavitation or nodular opacities in the upper and posterior lung segments, and in the appropriate clinical setting, a specific diagnosis of pulmonary TB can be established17. While chest CT is useful for clarifying confusing findings, it has not been conclusively shown to have a significant impact on patient management14, therefore microbiological identification of TB by culture should follow this test11.

For pulmonary TB, sputum is the most critical sample for laboratory testing. Direct sputum smear microscopy is the most widely used method for diagnosing pulmonary TB and is available in most primary health-care laboratories at the health-center level3,12. Smear microscopy may, however, be costly and inconvenient for patients, who must make multiple visits to health facilities and submit multiple sputum specimens over several days. Fortunately, good-quality microscopy of two consecutive sputum specimens has been shown to identify the vast majority (95%-98%) of smear-positive TB patients18,19. The WHO policy on case detection by microscopy was, therefore, revised to recommend a refnew TB diagnostics on both duction in the number of specimens examined, from three to two in settings with appropriate external quality assurance and documented high-quality microscopy18. In addition, the WHO recommends that countries that have successfully implemented current WHO policies for a two-specimen case-finding strategy consider switching to same-day diagnosis, especially in settings where patients are likely to default from the diagnostic process18. The Korean guideline for TB recommends that presumptive TB patients should have multiple sputum samples, at least two, possibly three, and allows first sputum sample for front-loaded or same-day microscopy11.

Conventional light microscopy of Ziehl-Neelsen-stained smears prepared directly from sputum specimens is the most widely available test for diagnosing TB in resource-limited settings. Ziehl-Neelsen microscopy is highly specific, but its sensitivity is variable (20%-80%). Conventional fluorescence microscopy is more sensitive (10%) than the Ziehl-Neelsen and takes less time, but it is limited by the high cost of mercury vapor light sources, the need for regular maintenance, and the dark room requirement20. Light-emitting diodes (LED) have been developed to offer fluorescence microscopy without the associated costs. The efficacy of LED microscopy was assessed by the WHO and evaluated by standards appropriate for the accuracy and the effect of new TB diagnostics on both patients and public health21. The results showed that the accuracy of LED microscopy was equivalent to that of international reference standards; it was more sensitive than conventional Ziehl-Neelsen microscopy, and it had qualitative, operational, and cost advantages over both conventional fluorescence and Ziehl-Neelsen microscopy. Based on these findings, the WHO recommends that conventional fluorescence microscopy be replaced by LED microscopy and that LED microscopy be phased in as an alternative for conventional Ziehl-Neelsen light microscopy21.

Laboratory diagnosis of TB relies on direct microscopic examination of sputum specimens. However, the technique, although specific, has low and variable sensitivity and cannot identify drug-resistant strains. Clinicians have been advised to obtain culture confirmation of TB whenever possible. This not only confirms the diagnosis, but also obtains material for crucial drug susceptibility testing (DST)3,9,12. Mycobacterial culture is more sensitive, but growth of TB bacilli on traditional solid medium requires 4-8 weeks, which delays appropriate treatment in the absence of a confirmed diagnosis. Culturing mycobacteria is mainly done on solid media, the Lowenstein-Jensen slope, or in broth media. These methods are slow, with cultures from microscopy-positive material taking from 2-4 weeks and for microscopy-negative material from 4-8 weeks. Therefore, liquid media remains the mycobacteriology gold standard for initial isolation, because it is significantly faster (between 10 and 14 days) and is better for isolation, compared to solid media. For DST, the delay may be reduced to as little as 10 days compared to 4-6 weeks with conventional solid media. Liquid systems are more sensitive for detecting mycobacteria and may increase the case yield by 10% over solid media. With increased sensitivity and reduced delays, liquid systems may contribute significantly to improved patient management. Liquid systems are, however, more prone to contamination by other microorganisms. In experienced laboratories, approximately 5%-10% of specimens fail to yield results because of contamination. Procedures to prevent cross-contamination (due to carryover of bacilli from positive to negative specimens) should also be strictly followed, especially where increased numbers of positive specimens are processed in high-incidence countries22. Solid media are made of agar, egg, and malachite green to limit the growth of remaining contaminants (Lowenstein, Stonebrink, or Ogawa medium). Liquid culture and DST systems are more complex and sensitive than solid culture and DST media. Increased bacterial contamination and an increased frequency of nontuberculous mycobacterial (NTM) isolation must be addressed. A rapid method to differentiate the Mycobacterium tuberculosis complex from other mycobacterial species is essential. Several manufacturers have recently marketed tools that can automatically detect M. tuberculosis growth in the laboratory, such as the Bactec "Mycobacterial Growth Indicator Tube 960" (MGIT 960; Becton-Dickinson, Sparks, MD, USA) and the MB/Bact Alert 10 3D (Biomérieux, Durham, NC, USA). Unfortunately, these automatized incubators are expensive, they do not give rapid mycobacterial species' identification, and they do not identify contaminated or mixed cultures. Conversely, cultures on solid media provide all of this information with a simple observation of colonies. The current guideline recommends that all specimens cultured on liquid media also be inoculated on solid media to ensure purity and sufficient strengthen for the diagnosis9,11.

People suspected of having TB should be referred for medical evaluation. A posterior-anterior chest X-ray should be taken, and a chest X-ray that appears suggestive of TB should be followed by further diagnostic investigation. Multiple sputum samples (at least two, possibly three samples) that allow at least, first one front-loaded sputum specimen should, if possible, be sent for TB microscopy and cultured for suspected respiratory TB before starting treatment11. Spontaneously produced sputum should be obtained if possible; otherwise, induction of sputum or bronchoscopy and lavage can be considered36,37.

NAA tests can rapidly confirm TB diagnosis and distinguish M. tuberculosis from NTM in a sputum smear-positive person. Positive results for both the NAA and the AFB smear tests strongly suggest that the patient has TB, and anti-TB treatment should be initiated while awaiting culture results. The PPV of FDA-approved NAA tests for TB in the United States is >95% in AFB smear-positive cases. If the NAA result is negative and the AFB smear result is positive, a patient can be presumed to have an NTM infection11,25. Culture remains the gold standard for laboratory confirmation of TB and is required for isolating bacteria for DST and genotyping11.

Current TB diagnostics are still suboptimal in performance, especially for smear-negative TB. Because TB can present with many different symptoms, the first obstacle in diagnosing smear-negative TB is discerning the varied clinical presentations, to determine which conditions are highly suspicious and should be included in the differential diagnosis. A diagnostic approach to an AFB smear-negative patient with possible TB includes, where available, a detailed medical history and clinical examination as well as radiological, microbiological, molecular, and histological investigations38,39. In patients suspected of having TB with smear-negative, culturing remains essential for diagnostic testing and should be included when evaluating patients suspected of having TB with negative sputum smears. When performed correctly, culture increases diagnostic sensitivity, which ideally results in earlier case detection. The state-of-the-art diagnostic path often uses the tests mentioned above in combination, and additional diagnostic tools/procedures such as induced sputum37, chest CT imaging14,17,40, bronchoscopy with lavage36,41, and lung tissue biopsy42,43 are sometimes performed.

NAA testing is recommended to perform the initial diagnosis of patients that are suspected to have TB, even in smear-negative patients. If the NAA result is positive and the AFB smear result is negative, the clinician must consider the case as TB positive and begin anti-TB treatment while awaiting culture results11,25. If sputum smears and NAA tests are negative, and TB is still suspected, cultures are the most sensitive tests for TB. Culture is therefore very useful for diagnosing both smear-negative TB and drug-resistant TB. Although new diagnostic methods can decrease turn-around time with considerable diagnostic accuracy and convenience, they are not a replacement for culture and conventional DST25,44,45,46. However, in certain TB cases that are caused by pauci-bacillary disease, where only a small number of M. tuberculosis organisms are present, cultures can be negative; in addition sampling error, or technical problems can occur. In cases where the culture is negative, the standard used to compare with the diagnostic test could be response to treatment, clinical features, or a positive culture in the future. A TB diagnosis in this population would likely be achieved on a case-by-case basis and as such, would probably not be reported in many studies9,38,39.

Diagnosing TB worldwide still consists of methods that are intended to isolate the pathogen, which is a major limitation when the mycobacterial load is low or the site of infection is not easily accessible. For these reasons, diagnosis of smear-negative TB is often delayed, and such a diagnosis is often made based on the clinical response to empiric anti-TB treatment without microbiological confirmation38,39,44. Diagnostic algorithms have been recommended in the absence of rapid, simple, and accurate diagnostic tools for smear-negative pulmonary TB11,38,39,47. Diagnosing smear-negative pulmonary TB is achieved with at least two adequate expectorated multiple sputum samples, with negative smears, chest radiography findings consistent with TB, and a lack of response to a number of days of broad-spectrum antibiotics while awaiting culture results. For such patients, sputum cultures should be obtained and active follow-up is needed25,44. There are several limitations to using diagnostic steps based on antibiotics response. Because the fluoroquinolones and aminoglycosides are active against the M. tuberculosis complex and may cause transient improvement in people with TB, they should be avoided in this diagnostic phase. Strict adherence to the sequential steps of the algorithm could delay appropriate treatment in patients with an illness that is worsening rapidly. Finally, studies have shown that patients with TB might respond, at least transiently, to empirical broad-spectrum antibiotics, a frequent element of diagnostic algorithms7,48. Recently, the WHO revised the TB definition to include clinically-diagnosed TB instead of smear-negative TB. A clinically-diagnosed TB case is one that does not fulfill the criteria for bacteriological confirmation but has been diagnosed with active TB by a clinician or other medical practitioner who has decided to give the patient a full course of TB treatment. Clinically-diagnosed cases subsequently found to be bacteriologically-positive (before or after starting treatment) should be reclassified as bacteriologically confirmed13.

Clinicians who manage patients with suspected TB should ensure that their diagnostic practices align with the guidelines for TB, and use sputum smear microscopy and culture to investigate adult patients with suspected active TB3,9,11. Novel methods allow for rapid diagnosis of active TB in patients with negative sputum smears for AFB and enable prompt, highly accurate identification of drug-resistant strains of M. tuberculosis directly from respiratory specimens. Some of the structural limits of current TB diagnostics are likely to be overcome by such new tools, but research is still needed. The success of implementing new technologies and the development of additional diagnostic approaches must take into account the diagnostic needs of each context (point-of-need testing approach) together with the logistic, economic and technical constraints that are present in the majority of intermediate-TB-burdened settings in Korea.