infectious diseases
Tuberculosis (TB)
Diagnosis of TB
Chest x-ray Acid-fast stain and culture Tuberculin skin test (TST) or interferon-gamma release assay (IGRA) When available, nucleic acid amplification test (NAAT) Pulmonary tuberculosis is often suspected based on one of the following: Chest x-rays taken while evaluating respiratory symptoms (cough lasting > 3 weeks, hemoptysis, chest pain, dyspnea), an unexplained illness, fever of unknown origin (FUO), or a positive Skin testing (TST) IGRA (interferon-gamma release assay) done as a screening test or during contact investigation Suspicion for TB is higher in patients who have fever, cough lasting > 2 to 3 weeks, night sweats, weight loss, and/or lymphadenopathy and in patients with possible TB exposure (eg, via infectious family members, friends, or other contacts; institutional exposure; or travel to TB-endemic areas). Initial tests are chest x-ray and sputum examination and culture. If the diagnosis of active TB is still unclear after chest imaging and sputum examination, TST or IGRA may be done, but these are tests for infection not active disease. NAATs (eg, polymerase chain reaction [PCR]–based) are rapid and can be diagnostic. Like most clinical tests, positive TB test results are statistically more likely to be false positives when the prior probability of TB infection is low (see also Understanding Medical Tests and Test Results ). Once TB is diagnosed, patients should be Diagnostic tests , and those with risk factors for In adults, a multinodular infiltrate above or behind the clavicle is most characteristic of active TB; it suggests reactivation of disease. It is best visualized in an apical-lordotic view or with chest CT. Middle and lower lung infiltrates are nonspecific but should prompt suspicion of primary TB in patients (usually younger) whose symptoms or exposure history suggests recent infection, particularly if there is pleural effusion. Calcified hilar nodes may be present; they may result from primary TB infection but may also result from Histoplasmosis in areas where histoplasmosis is endemic (eg, the Ohio River Valley). Sputum testing is the mainstay for diagnosis of pulmonary tuberculosis. A nonsputum-based diagnostic test has long been sought because sputum is often difficult to collect; breath and urine tests are available, and the urine tests have proved useful in diagnosing TB disease in people with HIV infection. If patients cannot produce sputum spontaneously, aerosolized hypertonic saline can be used to induce it. If induction is unsuccessful, bronchial washings, which are particularly sensitive, can be obtained by fiberoptic bronchoscopy. Because induction of sputum and bronchoscopy entail some risk of infection for medical staff, these procedures should be done as a last resort in selected cases. Appropriate precautions (eg, negative-pressure room, N-95 or other fitted respirators) should be used. The first step in sputum examination is typically microscopic examination to check for acid-fast bacilli (AFB). Tubercle bacilli are nominally gram-positive but take up Gram stain inconsistently; samples are best prepared with Ziehl-Neelsen or Kinyoun stains for conventional light microscopy or fluorochrome stains for the more sensitive fluorescent microscopy. Smear microscopy can detect about 10,000 bacilli/mL of sputum, making it insensitive when fewer bacilli are present, as occurs in early reactivation or in patients with HIV coinfection. Although finding AFB in a sputum smear is strong presumptive evidence of TB in the presence of TB risk factors, in other settings environmental mycobacteria may be more likely, and definitive diagnosis requires a positive mycobacterial culture or NAAT. Culture is also required to isolate bacteria for conventional drug susceptibility testing and genotyping. However, molecular drug susceptibility testing is increasingly replacing culture-based methods. Culture can detect as few as 10 bacilli/mL of sputum and can be done using solid or liquid media. However, it can take up to 3 months for final confirmation of culture results. Liquid media are more sensitive and faster that solid media, with results available in 2 to 3 weeks. Rapid antigen testing to detect the MPB64 antigen can confirm that organisms growing on mycobacterial culture areM. tuberculosis. Two types of NAAT are available for TB diagnosis: Xpert MTB/RIF Line probe assay (rifampicin) in as little as 2 hours. The Xpert MTB/RIF NAAT is more sensitive than sputum smear microscopy and about as sensitive as culture for diagnosing TB. Newer versions of Xpert MTB/RIF Ultra (Xpert Ultra) can detect resistance mutations to both First-line drugs for TB and many Theline probe assaycan identify the presence ofM. tuberculosisand resistance torifampinandisoniazid. However, sensitivity is lower than that of Xpert MTB/RIF. This test is usually done only on smear-positive specimens. Probes for several second-line drugs are available. There are a variety of diagnostic algorithms that differ based on the tests available. If an Xpert MTB/RIF or Xpert Ultra test on a sputum sample is positive, the diagnosis of pulmonary TB is considered confirmed. In such cases, treatment can be started based on local drug susceptibility patterns. If NAAT and AFB smear results are negative or if AFB smear results are positive and NAAT results are negative, clinical judgment is used to determine whether to begin anti-TB treatment while awaiting results of culture. Drug susceptibility tests should be done on initial isolates from all patients to identify an effective anti-TB regimen. These tests should be repeated if patients continue to produce culture-positive sputum after 3 months of treatment or if cultures become positive after a period of negative cultures. TB disease can be caused by more than one strain, each with different drug resistance patterns, especially in settings of high transmission. Results of susceptibility tests may take up to 8 weeks if conventional bacteriologic methods are used, but as noted above, several molecular DSTs can rapidly (within hours) detect drug resistance torifampinand other first- and second-line drugs. Transbronchial biopsies can be done on infiltrative lesions, and samples are submitted for culture, histologic evaluation, and molecular testing. Gastric washings, which are culture-positive in a minority of samples, are no longer commonly used except in small children, who usually cannot produce a good sputum specimen. However, sputum induction is being used in young children who are able to cooperate. Ideally, biopsy samples of other tissue should be cultured fresh. Culture of fresh specimens remains the gold standard for the detection ofM. tuberculosisin tissue because fixation can inhibit PCR, and PCR does not distinguish viable from dead organisms. However, NAAT can be used for fixed tissues when necessary (eg, for biopsied lymph node if histologic examination unexpectedly detects granulomatous changes). This use of NAAT has not been approved, but it can be extremely useful, although positive and negative predictive values have not been established. Multiple-puncture devices (tine test) are no longer recommended. The tuberculin skin test (TST—Mantoux intradermal method) using purified protein derivative (PPD) is usually done. The TST measures the immunologic response toM. tuberculosisand thus should be positive in both latent and active infection and so cannot distinguish between the two. The standard dose in the US of 5 tuberculin units (TU) of PPD in 0.1 mL of solution is injected on the volar forearm. It is critical to give the injection intradermally, not subcutaneously. A well-demarcated bleb or wheal upon injection indicates a properly placed injection. The diameter of induration (not erythema) transverse to the long axis of the arm is measured 48 to 72 hours after injection. Use of a pen to demarcate the boundaries of induration on the skin can help produce more precise measurements, but reading skin tests is inherently variable and subject to a number of errors, including terminal digit preference, that is, a tendency to favor recording 5-, 10-, 15- and 20-mm results. In research studies, measurements done with a caliper or ruler where measurement numbers were not immediately visible to the reader produced less biased readings. Given the difficulty of demarcating and accurately measuring the induration, it is ill-advised to attach clinical significance to minor differences. For example, a 9-mm reading should probably not be interpreted as different from an 11-mm reading (ie, treating the 11-mm reading as latent infection while dismissing the 9-mm reading as uninfected). Among household contacts, and in other settings where recent transmission is considered certain, TST results average about 17 mm in induration. Clinically, it is useful to remember that recently infected people are at greatest risk of reactivation and that, if they are immunocompetent, they usually have a vigorous immune response, manifested by a large TST or interferon gamma release test ( IGRA (interferon-gamma release assay) ) result. With the tuberculin skin test, it is the diameter of induration, not erythema, that is measured. TST responses tend to decrease over time, commonly long outlasting the presence of viableM. tuberculosisorganisms capable of reactivation. Although late reactivation is well documented, most reactivation of latent infection occurs within a year to 18 months of initial infection. Treating latent TB many years after infection likely occurred may be advisable when immunosuppression is contemplated, but residual infection that is likely to reactivate may no longer be present. Reversion of skin tests that occurs in the absence of treatment or anergy (no reaction to any skin test) is often missed because follow-up testing is not done. Spontaneous cure is the likely reason. In settings of high transmission, disease often results fromrecentrather thanremoteinfection, although both can occur. Repeated administration of TSTs can cause the immune system to recall previous hypersensitivity that has waned over time, so-called boosting. Unrecognized, boosting can result in unnecessary treatment of contacts, for example, in the context of an outbreak investigation. To avoid misinterpreting boosting as recent infection in settings where serial testing is indicated, two-step baseline testing is recommended. The idea is to retest people with a negative TST result within a 1 to 4 weeks to see whether there is recall of previous hypersensitivity. If not, that result is a true negative. However, a positive TST result on retesting 1 to 4 weeks after the first test is assumed to indicate pre-existing latent infection and patients are treated or not treated based on clinical criteria. Boosting is not a problem with repeated IGRA testing because no antigens are injected. Recommended cutoff points for a positive TST reactiondepend on the clinical setting: 5 mm:Patients at high risk of developing active TB if infected, such as those who have chest x-ray evidence of past TB, who are immunosuppressed because of HIV infection or drugs (eg, TNF-alpha inhibitors, corticosteroid use equivalent toprednisone15 mg/day for>1 month), or who are close contacts of patients with infectious TB 10 mm:Patients with some risk factors, such as injection drug users, recent immigrants from high-prevalence areas, residents of high-risk settings (eg, prisons, homeless shelters), patients with certain disorders (eg, silicosis, renal insufficiency, diabetes, head or neck cancer), and those who have had gastrectomy or jejunoileal bypass surgery 15 mm:Patients with no risk factors (who typically should not be tested) False-negative TST resultscan occur, most often in patients who are febrile, older, HIV-infected (especially if CD4 count is<200 cells/mcL [0.2 x 109/L]), immune-suppressed because of a disorder or use of certain drugs (eg, corticosteroids, certain biologic immune modulators, certain cancer drugs), or very ill. Many of these people show no reaction to any skin test (anergy). Anergy probably occurs because inhibiting antibodies are present or because so many T cells have been mobilized to the disease site that too few remain to produce a significant skin reaction. may occur if patients have Nontuberculous Mycobacterial Infections or have received the bacille Calmette-Guérin (BCG) vaccine. However, the effect of BCG vaccination on TST usually wanes after several years; after this time, a positive test is likely to be due to TB infection. Infection after BCG vaccination occurs in high-transmission settings. People arriving in the US from high-burden areas often cite BCG as the reason for their positive TST, rather than get stigmatized by a diagnosis of TB, and resist treatment for latent infection even when it is clearly indicated. Authorities suggest that BCG vaccination status should be ignored and infection assumed, but this leads to overdiagnosis of latent TB infection, unnecessary concern and treatment, and potential drug adverse effects. The use of IGRAs to diagnose TB infection, although not also without interpretation problems, has largely solved the BCG/latent TB infection controversy. Two commercial IGRAs with different testing platforms exist, but many comparative studies reveal very similar performance. The choice of IGRA is often dictated by availability in a given clinical setting. The IGRA is a blood test based on the release of interferon gamma by lymphocytes exposed in vitro to TB-specific antigens. Although results of IGRAs are not always concordant with TST, these tests appear to be as sensitive as and more specific than TST in contact investigations. Unlike the TST, prior BCG vaccination does not cause a false-positive test result on IGRA, but repeat TST testing itself can cause low-level positive IGRA results. Like the TST, IGRAs do not distinguish active from latent TB and although lymphocyte hypersensitivity occurs in both conditions, because in vitro lymphocyte stimulation with TB antigens is brief, IGRAs are thought more likely to reflect recent (sensitized circulating lymphocytes) rather than remote (sensitized memory lymphocytes) TB infection. Therefore, IGRAs are thought to better correlate with a risk of reactivation compared to the TST. In low-TB burden areas where more resources are available to focus on latent infection, IGRAs have largely replaced the TST because they have less operator-related variability, lack BCG interference, lack Booster reaction , and people do not have to return for a reading. However, in low-TB burden areas like the US, fewer and fewer people are at sufficient risk to warrant even one-time much less serial TB testing. Use of IGRA is limited in low-resource settings because of its relatively high cost. However, the WHO is emphasizing the treatment of latent infection in high-TB burden as a necessary component of TB elimination globally. Because of BCG interference with the TST, IGRAs are clearly more specific for latent TB. 