5 курс / Пульмонология и фтизиатрия / Clinical_Tuberculosis_Friedman_Lloyd_N_,_Dedicoat

There are a number of operational limitations of sputum collection for TB diagnosis. Most important are infection control concerns, as expectorating sputum produces cough aerosols which may lead to nosocomial spread of TB. Thus, health workers should wear personal protective equipment such as N95 particulate respirators, and, where resources allow, sputum and respiratory samples should be collected in rooms equipped with negative-pressure ventilation and high-efficiency particulate exhaust systems; sterilization of the air with ultraviolet germicidal irradiation (UVGI) following the procedure adds an additional measure of protection.50 If such facilities are not available, specimens should be collected away from other patients where there is adequate natural ventilation and plenty of sunlight. Second, not all patients are able to expectorate sputum, especially young children. At least 1–5 mL of good quality, non-salivary sputum, should be collected, but the best available specimen should be analyzed.38 In individuals who cannot expectorate on an initial request and with coaching, it is worth making a second request for sputum at a later time, before moving on to sputum induction or bronchoscopy.

Sputum induction

In patients unable to produce sputum after coughing, a 15-minute treatment with hypertonic saline aerosolized using an ultrasonic nebulizer or pneumatic compressor and delivered via high-flow air through a mouthpiece is a well-established procedure to induce coughing and expectoration of a sample from the lower airways. Usually administered in concentrations of 3%–7% NaCl, the aerosolized saline settles in the central airways where it promotes osmotic inflow of interstitial fluid into the respiratory tract. This additional fluid appears to trigger cough via the airway receptors, leading the patient, with encouragement from the supervising health worker, to expectorate thin fluid with or without mucus that may be sent for microbiologic examination.51 Laboratory staff should be notified via the laboratory requisition if a sample is induced to help prevent it from being inappropriately rejected as a salivary specimen. Infection control is a particular concern with sputum induction because the patient is asked to cough repeatedly over several minutes and should be carried out with careful attention to infection control measures.

Bronchoscopy

Fiberoptic bronchoscopy is a semi-invasive examination that allows inspection of the central airways and collection of lower samples via bronchial wash, bronchoalveolar lavage (BAL), endobronchial brushings, endobronchial biopsy, transbronchial needle aspiration of enlarged paratracheal, mediastinal, and hilar lymph nodes in accessible locations with or without ultrasound guidance, and/or transbronchial lung biopsy. Samples may frequently be sent for cell counts, cytological staining, and/or flow cytometry, as well as microbiological examinations including AFB smear microscopy, NAAT, and mycobacterial culture. Airway inspection may rarely identify granulomatous lesions of the endobronchial mucosa, and transbronchial aspirates and biopsies may occasionally identify tissue that confirms active tuberculosis, or an alternative diagnosis such as infection

due to nontuberculous mycobacteria or fungal infection, an inflammatory condition such as sarcoidosis, or malignancy. In some clinical scenarios, such as when immunocompromised patients are undergoing evaluation, bronchoscopy may be indicated to obtain samples for diagnosis of other infections. In these circumstances, the procedure is often guided by chest CT to identify radiographic patterns and locations of disease that may inform the differential diagnosis and the types of samples to be collected. In such scenarios, bronchoscopy may offer the opportunity to obtain concentrated saline washings that can reduce the time to diagnosis of TB or other infections, and lead to management changes.52,53 In other scenarios, TB may be diagnosed unexpectedly as part of a diagnostic evaluation for another condition causing pulmonary infiltrates, cavities, or nodules, such as possible lung cancer. One particular group in whom bronchoscopy may be beneficial are patients in whom radiography shows a “miliary” pattern, characterized by diffuse 2–3 mm nodular opacities in a random distribution; these are said to resemble millet seeds and represent hematogeneously disseminated mycobacteria. Given this wide distribution of the bacilli via the vasculature, sputum samples tend to be paucibacillary and microbiologic examination is insensitive; endobronchial brushings, transbronchial biopsy, and/or bronchoalveolar lavage may confirm the diagnosis in more than half of patients with miliary TB.54 Outside these select scenarios, studies among immunocompetent and among immunocompromised patients have found no difference in the diagnostic yield of induced sputum and BAL whether examined by either smear microscopy or mycobacterial culture.55,56

There are some clinical and operational considerations that influence the feasibility of bronchoscopy, most notably the respiratory status of the patients. In spontaneously breathing patients, bronchoscopy requires topical anesthetization of the upper and lower airway, and most patients also require conscious sedation with benzodiazepines and opiates for anxiolysis and control of dyspnea, discomfort, and coughing, all of which can make the procedure uncomfortable for the patient and therefore technically challenging for the proceduralist. Given the potential risks of over-sedation and respiratory failure, bronchoscopy is undertaken in a monitored setting. Patients with severe respiratory distress may require intubation and mechanical ventilation for safety, and the risk of clinical complications including pneumothorax (e.g. if transbronchial biopsy is performed) and worsening hypoxemia must be weighed against the risk of continuing care without a confirmed diagnosis.

CURRENT SPUTUM TESTS

Smear microscopy

Sputum smear microscopy dates back to Robert Koch’s seminal discovery of M. tuberculosis in 1882 and remains the most common diagnostic test for TB. Usually performed on sputum, the test involves directly smearing a small sample of diagnostic material (sputum, fine-needle aspirate, or other body fluid) onto a disposable glass slide, heat-fixing the material to the glass,

and applying chemical dyes that are specific for mycobacteria and a small number of other bacteria, including Nocardia and Rhodococcus. The classic carbol-fuchsin stains exploit a unique property of the mycobacterial cell wall that allows it to absorb the brilliant fuchsia-colored dye. They retain this coloring even after application of a background methylene blue counterstain and a subsequent destaining procedure with acid alcohol. Under a light microscope, deep pink bacilli may be readily viewed against the blue background and counted as semi quantitative measure of bacillary load. A limitation of the technique is that it requires the technician to view at least one 2-cm slide length under a 1000× objective over 10 minutes in order to exclude smear-positive TB.38,57 Newer microscopes using long-lasting, low-cost LED bulbs have been adapted to visualize fluorochrome stains of the mycobacterial cytoplasm, which permit viewing at 200× and greatly reduce the time required to read a slide.58 Nonetheless, sputum smear microscopy is limited by poor sensitivity, detecting only 50%–70% of active pulmonary TB patients. Moreover, while the technique has high specificity in settings with high TB prevalence, its specificity and positive predictive value are significantly reduced in low TB-prevalence settings, and among patients with a history of prior TB. Using mucolytic agents and centrifugation prior to processing have been shown to increase the sensitivity of smear microscopy by an average of 13%.59 Processing methods that do not require centrifugation (e.g. bleach digestion) have also been described, but are associated with modestly reduced specificity.60

Mycobacterial culture

Mycobacterial culture of two-to-three respiratory samples on solid or liquid media is considered the reference standard test for diagnosis of pulmonary TB. Unfortunately, mycobacterial culture requires substantial technical expertise and laboratory infrastructure, including level 3 biosafety protection, because a high concentration of bacilli in culture increases the risks of M.tb transmission to laboratory staff. As a result, mycobacterial culture is usually available only in centralized commercial or public health laboratories and is not routinely available in most lowincome countries.

Several factors related to specimen collection and processing have a major influence on the sensitivity and specificity of mycobacterial culture. For example, because the quantity of bacilli may vary from sample to sample even within the same subject, at least three specimens should be sent for culture if resources permit. Oral commensal bacteria contaminate sputum during expectoration, and can overgrow the culture medium within days, preventing mycobacteria from growing. To limit this, laboratories decontaminate sputum using sodium hydroxide (1%– 4% concentration) prior to inoculation in culture, but this may eliminate up to a third of all mycobacteria in the sample. As with microscopy, treatment with mucolytic agents such as N-acetyl- l-cysteine and concentration via centrifugation helps maximize the number of mycobacteria for inoculation.36 In spite of these measures, around 5% of all sputa sent for mycobacterial culture will deliver an indeterminate result because of contamination. To reduce the probability of contamination, sputum sent for culture

should be stored and transported via cold chain and processed as rapidly as possible upon arrival in the laboratory. Alternatively, sputum may be treated with agents such as cetylpyridium chloride and other newer commercial agents designed to stabilize and protect mycobacterial cells from overgrowth without cold chain requirements.61

There are a number of different choices of media, each with advantages and disadvantages. Solid media including Lowenstein−Jensen and Ogawa media are low-cost, egg-based media that are often prepared on site in laboratories, especially in low-income countries. Newer agar-based commercial media include Middlebrook 7H10 and selective 7H11 media. Newer liquid media systems, often combined with automated, in-tube colorimetric reading, include Middlebrook 7H9 media, now commonly supplied in mycobacterial growth indicator tubes (MGIT), a colorimetric system that can be measured manually or using automated systems for early detection and reporting. In addition to using glycerol and other inorganic compounds as a primary nutrient source, liquid media are supplemented with oleic acid−albumin−dextrose−catalase (OADC) and the antibiotics polymyxin B, amphotericin B, nalidixic acid, trimethoprim, and azlocillin (PANTA) to select against more rapidly growing bacteria. Compared with solid culture, liquid culture is fosters more rapid growth of mycobacteria. Median time to positivity is 13 days, and cultures are considered negative if no bacteria are detected after 42 days of monitoring.62 In contrast, growth on solid media is expected after a median of 26 days, and cultures are considered negative after 56 days. Liquid media provide greater sensitivity and shorter turn-around time for detection of mycobacteria, albeit at the cost of more frequent contamination by bacteria and nontuberculous mycobacteria. Solid culture is lower in cost, and the visualization of discrete mycobacterial colonies may also allow identification of multiple species or strains in the same specimen. Overall sensitivity of MGIT liquid media is 88% and specificity 99.6%, as compared with a sensitivity of 76% and specificity of 99.9% for solid media.62

Interestingly, there appears to be a degree of selectivity to all media, and, if resources allow, every specimen should be cultured on at least one solid and one liquid medium to minimize the consequences of selectivity and contamination.63 Once a culture is noted to be positive, additional tests must be performed for speciation and drug susceptibility testing. Traditionally, selective culturing and biochemical assays were used to identify species of mycobacteria, but increasingly, nucleic acid amplification, MPT 64 antigen testing using lateral flow assays, and high-performance liquid chromatography are used for identification.

Nucleic acid amplification testing

NAAT for TB diagnosis first became widely available in laboratories in the early 1990s, with the commercial introduction of assays targeting the IS6110 sequence.46 Although widely endorsed by public health authorities,64,65 first-generation assays never achieved widespread adoption67 for a variety of clinical and operational reasons. Although there were many high-quality diagnostic accuracy studies showing NAAT to be highly sensitive and specific for TB,66 as well as thoughtful clinical and public health

guidance about how to integrate test results with clinical decisionmaking,19,68 there were very few studies demonstrating an important clinical impact.69 On the contrary, studies from both the United States70 and the United Kingdom71 showed that clinicians were comfortable initiating therapy for positive results, but rarely withheld therapy in the setting of negative results. Operationally, these assays were complex to perform, requiring up to 8 hours of dedicated laboratory technician time and complicated, multiroom, unidirectional work flows. The open-tube nature of these assays made them susceptible to laboratory cross-contamination, and they developed a reputation for false-positive results.72 Given their imperfect sensitivity, there were also concerns about falsenegative results, and CDC guidelines at that time recommended repeat testing if the clinical probability of active TB remained high.68 Given low demand for first-generation NAATs from clinicians and concerns from laboratory directors that the assays were not affordable, availability of the tests decreased and turn-around time increased in high-income countries;73 they were never routinely introduced in low-income countries because of high costs and intensive labor requirements.

In 2010, the first clinical data on next-generation NAATs was reported, launching a new era in TB diagnostics.74–76 Unlike previous NAATs, these tests were developed specifically to address the need for rapid, low-cost assays to replace smear microscopy in lowand middle-income countries, through the novel mechanism of public−private partnerships. The first of these assays, GeneXpert MTB/RIF, applied several major technological advances to NAATs: (1) use of a novel, amplification-compatible mucolytic reagent and ultrasonic cell lysis to optimize extraction of target nucleic acids; (2) use of hemi-nested nucleic-acid amplification to improve sensitivity; (3) targeting of rpoB, a highly conserved 81 base-pair region of the Mtb genome to simultaneously provide information about TB diagnosis and about putative drug resistance to rifamycins; (4) amplification inside a closed, disposable cartridge and use of short, overlapping molecular beacon probes to improve specificity; and (5) automation of amplification using microfluidics to reduce labor requirements and turn-around time. Multiple clinical studies have shown that Xpert MTB/RIF detects virtually all sputum smear-positive TB patients and the majority of sputum smear-negative patients in most settings in both highand low-burden countries. In 70 studies of 37,237 possible TB patients included in a Cochrane review, pooled sensitivity of Xpert MTB/RIF was 85% and pooled specificity 98%; in 48 studies enrolling 8020 participants, pooled sensitivity for rifampicin resistance was 96% and specificity 98%.77

Following WHO approval in 2011,78 Xpert MTB/RIF was introduced widely in low-income countries. While it was originally targeted to persons living with HIV and those with risk factors for MDR-TB, with expansion of concessionary pricing through donor subsidies, WHO subsequently updated its policy to recommend Xpert as the first-line replacement test for smear microscopy for all forms of TB in all individuals, including children.79 Several postimplementation studies using quasi-randomized designs have shown that Xpert can increase the proportion of individuals with microbiologically confirmed diagnoses and shorten the time-to- treatment initiation. However, studies that have examined more important patient outcomes such as morbidity and mortality,

have failed to show an important impact in routine practice.80–82 For example, the TB-NEAT trial, a multi-country, individually randomized trial comparing nurse-performed Xpert MTB/RIF to standard sputum microscopy in several countries in southern Africa showed no change in TB-related morbidity.82 Similarly, the XTEND study, a parallel-group, cluster-randomized trial of 4656 patients nested into the national roll-out of Xpert MTB/RIF at 20 clinics in South Africa, showed no difference in six-month mortality, with a substantial proportion (16%) of Xpert-positive TB patients never receiving their results or initiating treatment.83 These findings were confirmed in an individual patient metaanalysis of five randomized trials, although among persons living with HIV, Xpert was shown to have a statistically significant 24% reduction in mortality. 84

There are several potential explanations for the apparent failure of Xpert to impact clinically important outcomes in the general population of patients undergoing evaluation for active TB. Methodologists have noted that improvements in diagnostics are rarely large enough to impact downstream clinical outcomes like mortality, especially for highly curable conditions like TB.85 Other contributing factors may include high rates of presumptive treatment of smear-negative individuals in the control arms of these trials. Observational studies have described (1) high rates of initial losses to follow-up among individuals assigned to Xpert testing as a result of difficulties that centralized laboratories experience in returning results to patients; (2) a frequent failure to refer all possible TB patients for Xpert testing and ensure that testing is completed; (3) confusion among clinicians about interpretation of Xpert results, particularly for drug resistance; and (4) technical failures of the tests and testing platforms. These failures may arise from rugged field conditions where dust, heat, power cuts, power surges, irregular supplies, and lack of regular maintenance and repair services may reduce availability of testing or increase the frequency of indeterminate results.86,87 These observations have suggested to many that designing smaller and simpler Xpert machines and placing them at microscopy facilities could be a less costly and more effective strategy for improving diagnosis and more effectively linking these patients to care.119 Thus, a variety of modifications to the Xpert test cartridge and platform are planned, including reducing the number of modules to one per device and integrating a solar-powered battery.

Xpert MTB/RIF has been shown to provide similar diagnostic accuracy results in low-burden settings as in an FDA-registration study that enrolled a majority of patients in the United States.88 This study supported approval of Xpert MTB/RIF for the indications of TB diagnosis and exclusion of infectious TB.89,90 In addition, a series of hypothetical trials that performed both conventional smear-microscopy and Xpert MTB/RIF in the United States demonstrated a substantial impact on clinical and health systems outcomes in a variety of settings. Among outpatients in an urban TB control program, these benefits included potential reductions in unnecessary empiric treatment for active TB, unnecessary contact investigations, and unnecessary evictions from congregate housing.91 Among inpatients placed in respiratory isolation while undergoing evaluation for possible active TB, studies have projected substantial reductions in length of stay in isolation rooms and in hospital,92–95 as well as in health-system

costs.96,97 More recently, the safety, efficiency, and cost benefits of Xpert following implementation as part of a triage algorithm for inpatients were confirmed prospectively in a large pragmatic, before-and-after study in a safety-net hospital.98 In spite of longstanding CDC recommendations that all possible TB patients be evaluated with nucleic-acid testing, less than half of patient evaluations meet this quality metric.65,99,100 Current guidelines state that possible TB patients may have respiratory isolation discontinued based on two negative Xpert results on expectorated or induced sputum collected at least eight hours apart.101

In 2017, the GeneXpert Ultra, an updated NAAT with increased sensitivity for active pulmonary TB, was approved by WHO, and is expected to gradually replace Xpert MTB/RIF in lowand mid- dle-income countries. Xpert Ultra provides enhanced sensitivity through several technological innovations on the original Xpert assay. These include targeting the multi-copy IS6110 and IS1081 sequences of the Mtb genome for amplification alongside the rpoB targets; conversion of the rpoB and IS6110 assays into fully nested amplification assays; and doubling the starting volume of the processed sputum entering the amplification chamber.102 In addition, the assay has been modified to use sloppy molecular beacon probes and melting curve analysis in order to provide more accurate rifamycin resistance results. In a multi-center diagnostic accuracy study in which all participants provided sputum for testing by both Xpert assays and serial mycobacterial culture, Xpert Ultra had 17% (95% confidence interval [CI] 10%–24%) higher sensitivity than Xpert MTB/RIF but 2.7% (95% CI 1.7%−3.9%) lower specificity; performance in detecting rifamycin resistance was similar.103 In post-hoc analyses, two sub-groups accounted for nearly all of the false-positive results: patients with a history of prior TB and individuals with trace-positive results on Xpert Ultra testing. Repeat testing of individuals with trace-positive results eliminated the specificity differences when compared with Xpert MTB/RIF; this approach is recommended in the final WHO statement. The cartridge innovations that enabled multiplexing of targets for the Ultra assay have also facilitated development of a separate cartridge for isoniazid, fluoroquinolone, and amikacin resistance for use as a reflex text for pre-XDR/XDR TB.

An alternative next-generation molecular test for TB is the loop-mediated isothermal amplification test (TB LAMP), approved by WHO in 2016 for use as a replacement test for microscopy in peripheral centers lacking infrastructure to perform GeneXpert testing. TB LAMP is a rapid, manual, closed-tube reaction that requires only a heating block, tubes, and reagents, and can be read with the naked eye, all making it about 25% less costly than Xpert MTB/RIF. Although TB LAMP has several potential advantages over microscopy, including enhanced sensitivity, it requires a manual, multi-step process that is more labor intensive than GeneXpert and does not provide information about drug resistance. Moreover, data on its performance are based on very low-quality evidence for key populations, including PLWH.

A large number of additional NAAT assays are in development, many with multi-analyte detection capabilities for nontuberculous mycobacteria as well as Mtb. Many of these have acquired approvals from European and US (FDA) regulatory authorities, and a few are scheduled for WHO review. These technologies are comprehensively described in the UNITAID TB Diagnostic

Landscape Report, which has been updated continually over the last few years.104

Other molecular diagnostics for drug resistance

In addition to the GeneXpert platform, a growing number of other technologies are available or in development for molecular drug susceptibility testing (DST). Together, these technologies offer hope for being able to decrease the cost and turn-around time for diagnosis of drug-resistant TB.104 A common challenge for these methods is limited data about the correlations among phenotypic DST, genotypic DST, and clinical outcomes. Expert guidelines therefore encourage that the reporting of molecular DST include evidence about the correlation between specific mutations and clinical outcomes to guide interpretation,105 and WHO continues to recommend that all patients undergo culture-based phenotypic drug-susceptibility testing as the reference test.1

Line-probe assays (LpA) employ a generic technology for interrogating pre-amplified Mtb sequences using fluorescent hybridization probes on disposable test strips.106,107 The results are easily interpretable with the naked eye. They have been harnessed for a variety of purposes, including TB diagnosis, identifying different species of mycobacteria in cultures, and targeted genotyping of drug-resistance mutations. The assays are rapid and inexpensive, but because of the limited analytic sensitivity of the hybridization probes, they must be applied to pre-amplified DNA. This requires that they be performed in advanced laboratories with infrastructure and human-resource capacity to extract and amplify DNA. These assays are performed as add-on tests after positive smear microscopy or mycobacterial culture have indicated the presence of adequate amounts of template DNA for amplification. LpA have achieved their greatest impact in screening for drug resistance to first-line agents (i.e., isoniazid and rifampin) and second-line (i.e., second-line injectable drugs and fluoroquinolones). First recommended by WHO in 2008, with updated recommendations issued in 2016,106,107 line-probe assays are routinely available countrywide as part of the standard public health testing algorithm in South Africa.104 They have contributed substantially to earlier and lower cost diagnosis of MDR TB, pre-XDR TB, and XDR TB in many countries with high burdens of drug resistance. The limitations of LpA are that each probe targets only a single resistance mutation and each strip can accommodate only a limited number of probes. Thus, multiple strips may be required to test for secondline drug resistance and the sensitivity of LpA may be limited for detecting resistance to drugs with multiple resistance alleles and may be unable to detect resistance for rare alleles.

Sequencing involves a diverse set of high-throughput methods with the ability to amplify DNA with higher sensitivity and specificity than other available methods. This higher resolution may facilitate identification of rare or unusual sequence variants that could be clinically important for a variety of indications including identifying rare or silent drug-resistance mutations; diagnosing mixed infections with different mycobacterial species or different Mtb strains, including strains with different resistance patterns; and revealing transmission links and their directionality. The main limitations of

sequencing methods are the current high cost and complexity and lack of standardization of the process, which requires not only laboratory proficiency, but also an advanced understanding of bioinformatics to align and normalize sequences. Nonetheless, with rapidly falling costs and increasing standardization of methods, many public health programs in high-income countries now offer sequencing routinely, including the US Centers for Disease Control, many state public health laboratories, and Public Health England.At least one analysis suggests that whole genome sequencing for TB may be highly cost-effective compared with the conventional approach of mycobacterial culture, reflex species identification, and phenotypic drug-susceptibility testing.108

NON-SPUTUM TESTS

In spite of the many available sputum assays for TB diagnosis, there is ongoing demand for a test for TB that would not require sputum. Blood and urine are commonly considered alternatives, with a number of commercial assays with a variety of targets in development. Breath is a novel specimen type that is also being explored by a number of research groups using a variety of targets, but is at an earlier stage of development compared to blood and urine. Commercial serological tests with both antigen and antibody targets have long been widely available, especially in the private sector in Asia and Africa.109 High-quality evidence demonstrates that these assays have poor sensitivity and specificity,110,111 and WHO has specifically recommended that they not be used.9 There is ongoing research by a number of groups working to develop next-generation approaches to serologic testing,112,113 but as yet there have been no clinical studies of assays ready to be made commercially available. Interferon-gamma release assays are commercially available tests for latent TB infection. A number of studies have evaluated their performance for active pulmonary TB on peripheral blood and found them insufficiently sensitive or specific for active pulmonary TB.114,115 Two recent studies have reported the use of gene-expression profiling based on wholeblood amplification of human RNA signatures, with most of the data coming from children.116 The only commercially available and approved non-sputum test for TB is the urinary lipoarabinomannan (LAM) antigen test.

Lipoarabinomannan assays

Antigen-based detection of LAM is a WHO-approved assay for detection of TB in patients with advanced HIV, including individuals with CD4 counts less than 100 cells/µL and individuals with severe illness regardless of CD4 count (defined by the presence of any one of the four danger signs, including respiratory rate >30/min, temperature >39°C, heart rate >120/min, or inability to walk unaided).10 LAM is a lipopolysaccharide released from mycobacterial cell walls and found in high concentrations in urine in persons living with advanced HIV. The assay uses a lateral-flow format, with the readout interpretable using automated optical reading devices or the naked eye, with very good inter-reader and intra-reader agreement. This may be because of the higher prevalence of disseminated TB, including renal TB, in these populations,

and from higher LAM clearance rates in the renal glomeruli of individuals with severe illness. The pooled sensitivity across six studies of 2402 persons living with HIV (median CD4 71–210) undergoing evaluation for symptoms suggestive of TB was 59% (95% CrI 43–77) and the pooled specificity was 78% (95% CrI 64–88). In a subset of five studies of 859 PLWH with CD4 ≤100, pooled sensitivity was 56% (95% CrI 41–70), and pooled specificity was 90% (95% CrI 81–95). The suboptimal specificity of the assay may reflect the fact that LAM is also found in other mycobacteria, and that standards based on sputum culture and clinical assessment used in diagnostic accuracy studies may fail to capture some patients with disseminated forms of TB (e.g., miliary TB). WHO recommends against the use of LAM as a screening test, but says that it may be used for diagnosis of PLWH with CD4  ≤200 or among individuals who are seriously ill.10 Although the diagnostic accuracy of urinary LAM is less than ideal, the approach of diagnosing TB using a urinary antigen is an important breakthrough in the field because of its simplicity and comparative biosafety. There is substantial ongoing research to enhance the capture of LAM and develop assays with improved analytic sensitivity in PLWH and others.104 For example, a novel urinary LAM assay using monoclonal antibodies and a novel silverbased technique for visualization has recently shown enhanced sensitivity and and similar specificity for TB among PLWH.120

CONCLUSION

Diagnosing TB remains one of the most formidable challenges in medicine, both because of the often indolent nature of the disease and because of more than a century without progress in improving testing. After over a century with very little progress in TB diagnostics, the last decade has brought enormous progress in the science, advocacy, and implementation of new tests and strategies. Much more is yet to be done to reach the more than 3 million people around the world who are still living with TB unbeknownst to themselves or public health authorities. By redoubling advocacy for increased funding and political engagement to address the disease, drawing on new scientific breakthroughs, and building on established principles of high-quality individualized assessment and patient-centered care, there is hope that we may substantially narrow and someday eliminate the diagnosis gap in TB.

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