Tuberculosis Workup

Obtain the following laboratory tests:

• Sputum for acid fast smear and culture
• Complete blood count (CBC)
• Chemistries, including alanine aminotransferase (ALT) or aspartate aminotransferase (AST)
• Alkaline phosphatase
• Total bilirubin
• Uric acid
• Creatinine
• HIV serology in all patients with tuberculosis (TB) and unknown HIV status

For congenital TB, the best diagnostic test is the examination of the placenta for pathology, histology, and culture. Mycobacterial blood cultures of the newborn may also be helpful. Treatment may be necessary until placental culture results are negative.

If chest radiography findings suggest TB and sputum smear is positive for acid-fast bacilli, initiate treatment for TB.

Ziehl-Neelsen staining of sputum is a simple 5-step process that takes approximately 10 minutes to accomplish. While highly specific for mycobacteria, this stain is relatively insensitive, and detection requires at least 10,000 bacilli per mL; most clinical laboratories currently use a more sensitive auramine-rhodamine fluorescent stain (auramine O).

Routine culture uses a nonselective egg medium (Lowenstein-Jensen or Middlebrook 7H10) and often requires more than 3-4 weeks to grow because of the 22-hour doubling time of M tuberculosis. Radiometric broth culture (BACTEC radiometric system) of clinical specimens significantly reduced the time (10-14 d) for mycobacterial recovery. Newer broth culture media and systems for isolation are available for use in clinical laboratories based on a fluorescent rather than a radioactive indicator. The indicator is inhibited by oxygen; as mycobacteria metabolize substrates in the tubes and use the oxygen, the tube begins to fluoresce.^[23]

Deoxyribonucleic acid (DNA) probes specific for mycobacterial ribosomal ribonucleic acid (RNA) identify species of clinically significant isolates after recovery. In tissue, polymerase chain reaction (PCR) amplification techniques can be used to detect M tuberculosis -specific DNA sequences and thus, small numbers of mycobacteria in clinical specimens.^{[24, 25]}

Extrapulmonary involvement occurs in one fifth of all TB cases, although 60% of patients with extrapulmonary manifestations of TB have no evidence of pulmonary infection on chest radiograph or sputum culture. Ocular TB can be especially difficult to identify, owing to its mimicry and its lack of accessible sampling; a high index of suspicion is required.

The hallmark of extrapulmonary TB histopathology is the caseating granuloma, consisting of giant cells with central caseating necrosis. Rarely, if ever, are any TB bacilli seen.

Altered mental status, neck stiffness, decreased level of consciousness, increased intracranial pressure, and cranial nerve involvement can indicate tuberculosis meningitis or tuberculoma. TB can directly seed the meninges and, if suspected, performing a lumbar puncture for evaluation of the cerebrospinal fluid is necessary. In addition, a tuberculoma can be substantiated based on an increase in intracranial pressure and computed tomography (CT) scanning/magnetic resonance imaging (MRI).

If vertebral involvement (Pott disease) or brain involvement is suspected, it is important to consider that a delay in treatment could have severe repercussions for the patient (compression of the spinal cord and/or paraplegia); further evaluation is necessary with CT scanning or MRI.

Tuberculin sensitivity

Tuberculin sensitivity develops 2-10 weeks after infection and usually is lifelong.

Multidrug-resistant TB

Symptoms and radiographic findings do not differentiate MDR-TB from fully susceptible TB. Suspect MDR-TB if the patient has a history of previous treatment for TB, was born in or lived in a country with a high prevalence of MDR-TB, has a known exposure to a MDR-TB case, or is clinically progressing despite standard TB therapy. Susceptibilities should be repeated if cultures remain positive after 2 months, even when initial susceptibilities did not reveal any resistance.

Pregnancy

Pregnancy provides an opportunity to screen for TB; all pregnant women can undergo tuberculin skin testing. If skin-testing results are positive, chest radiography can be performed with lead shielding. Chest radiography should not be delayed during the first 3 months of pregnancy in patients with suggestive symptoms.

TB in children

Postnatal TB is contracted via the airborne route. The most common findings of postnatal TB include adenopathy and a lung infiltrate. However, the chest radiographic findings may be normal in infants with disseminated disease.

Chest radiographs in children with TB may show only hilar lymphadenopathy or a patchy infiltrate. Most children with TB can be treated with isoniazid and rifampin for 6 months, along with pyrazinamide for the first 2 months if the culture from the source case is fully susceptible. Gastric aspirates or biopsies are not necessary if positive cultures have been obtained from the source case.

Go to Pediatric Tuberculosis for complete information on this topic.

Human immunodeficiency virus

Individuals infected with HIV are at increased risk for TB, beginning within the first year of HIV infection.^[26] Based on historical data, the initiation of antiretroviral therapy (ART) decreases the risk of developing TB in these patients.^[27] Risk for TB remains higher the first 3 months after starting ART; risk was highest among patients with a baseline CD4 count of less than 200/μL, higher baseline HIV-1 RNA level (relative hazard 1.93 for every log increase in baseline HIV-1 RNA), a history of injection drug use, and male sex.^[28]

In a study from Durban, South Africa, nearly 20% of patients starting ART had undiagnosed, culture-positive pulmonary TB. Neither cough nor acid-fast bacillus smear were sufficiently sensitive for screening. TB sputum cultures should be attempted before ART initiation in areas with a high prevalence of TB.^[29]

Patients with TB must be tested for HIV, and patients with HIV need periodic evaluation for TB with tuberculin skin testing and/or chest radiography. Patients with HIV and a positive tuberculin skin test result develop active TB at a rate of 3-16% per year.

Patients with TB and HIV are more likely to have disseminated disease and less likely to have upper-lobe infiltrates or classic cavitary pulmonary disease. Patients with a CD4 count of less than 200/μL may have mediastinal adenopathy with infiltrates.

Patients suspected of having tuberculosis (TB) should submit sputum for smear and culture. Sputum should be collected in the early morning on 3 consecutive days. In hospitalized patients, sputum may be collected every 8 hours. However, the absence of a positive smear result does not exclude active TB infection.

Approximately 35% of culture-positive specimens are associated with a negative smear result.

In patients without spontaneous sputum production, sputum induction with hypertonic saline should be attempted.^[30] Early-morning gastric aspirate may also produce a good specimen, especially in children.

Patients diagnosed with active TB should undergo sputum analysis for M tuberculosis weekly until sputum conversion is documented. Monitoring for toxicity includes baseline and periodic liver enzymes, CBC count, and serum creatinine.

Another option is fiberoptic bronchoscopy with transbronchial biopsy and bronchial washings. Biopsy of bone marrow, liver, or blood cultures is occasionally necessary and may be helpful.

Traditional mycobacterial cultures require weeks for growth and identification. Newer technologies, including ribosomal RNA probes and DNA PCR, allow identification within 24 hours. The DNA probes are approved for direct testing on smear-positive or smear-negative sputa. However, smear-positive specimens yielded higher sensitivity.

Culture for acid-fast bacilli (AFB) is the most specific and allows direct identification and susceptibility of the causative organism; however, access to the organisms may require lymph node/sputum analysis, bronchoalveolar lavage, or aspirate of cavity fluid or bone marrow. Unfortunately, obtaining the test results is slow (3-8 wk), and they have a very low positivity in some forms of disease.

AFB stain is quick but requires a very high organism load for positivity. This is more useful in patients with pulmonary disease, but a delay in diagnosis can increase mortality, as other diagnostic testing may need to be considered.

Blood cultures using mycobacteria-specific, radioisotope-labeled systems help to establish the diagnosis of active TB. Mycobacterial bacteremia (bacillemia) is detectable using blood cultures only if specialized systems are used. The bacilli have specific nutrient growth requirements not met by routine culture systems.

Such blood cultures should be used for all patients with HIV who are suspected of having TB, because bacillemia is particularly prevalent in this population. If available, such cultures should be used for any patient highly suspected of having active TB. One study found an incidence of 88% mycobacterial infection (66% TB, 22% Mycobacterium avium complex [MAC]) detected by blood culture in stage IV HIV disease).

Because conventional drug susceptibility tests for drug-resistant M tuberculosis take at least 3-8 weeks, Choi et al recommend direct DNA sequencing analysis as a rapid and useful method for detecting drug-resistant TB. In their clinical study of the use of direct DNA sequencing analysis for detecting drug-resistant TB, turnaround time of the direct DNA sequencing analysis was 3.8 +/- 1.8 days.

A total of 113 sputum specimens from 111 patients in the study were tested for genes conferring resistance to isoniazid, rifampin, ethambutol, and pyrazinamide, and the results were compared with drug susceptibility tests. The sensitivity and specificity of the assay were 63.6% and 94.6% for isoniazid, 96.2 and 93.9% for rifampin, 69.2% and 97.5% for ethambutol, and 100% and 92.6% for pyrazinamide, respectively.^[31]

An automated molecular test that uses sputum samples for the detection of M tuberculosis and resistance to rifampin has been developed. In studies conducted in low-income countries, the sensitivity for TB was 98.3% (CI, 97-99%) using a single smear-positive sputum sample and 76.9% (CI, 72.4-80.8%) using a single smear-negative sputum sample. Sensitivity from smear-negative sputum samples increased to 90.2% when 3 samples were tested. The test correctly identified 94.4% (CI, 90.8-96.6%) of rifampin-resistant organisms and 98.3% (CI, 97.1-99%) of rifampin-sensitive organisms.^{[32, 33]}

Microscopic-observation drug susceptibility (MODS) and thin-layer agar (TLA) assays are inexpensive, rapid alternatives to conventional methods or molecular methods for TB drug susceptibility testing. WHO endorsed the MODS assay, as a direct or indirect test, for rapid screening of patients with suspected MDR-TB. The evidence is insufficient to recommend the use of the TLA assay for rapid screening, but this assay is a promising diagnostic technique. Further research is encouraged.^[34]

Obtain a chest radiograph to evaluate for possible associated pulmonary findings (demonstrated in the images below). A traditional lateral and PA view should be ordered. In addition, an apical lordotic view may permit better visualization of the apices and increase the sensitivity of chest radiography for indolent or dormant disease.

The chest film is also useful to screen for sarcoidosis, which closely imitates the clinical course of ocular TB. Radiologists look more decisively for signs of TB or sarcoid if the requesting physician simply asks to rule out sarcoid or TB.

Chest radiographs may show a patchy or nodular infiltrate (as seen in the image below). TB may be found in any part of the lung, but upper-lobe involvement is most common. The lordotic view may better demonstrate apical abnormalities.

Primary TB is more likely to mimic the appearance of routine community-acquired pneumonia on chest radiography, in contrast to reactivation TB. Studies have shown that either may be associated with pleural effusion or cavitation.

Various patterns may be seen, as follows (these are further discussed below):

• Cavity formation - Indicates advanced infection and is associated with a high bacterial load
• Noncalcified round infiltrates - May be confused with lung carcinoma
• Homogeneously calcified nodules (usually 5-20 mm) - Tuberculomas; represent old infection rather than active disease
• Miliary TB - Characterized by the appearance of numerous small, nodular lesions that resemble millet seeds on chest radiography (Go to Miliary Tuberculosis for complete information on this topic.

Chest radiography (see the image below) consistent with TB indicates active disease in the symptomatic patient even in the absence of a diagnostic sputum smear. Similarly, normal chest radiographic findings in the symptomatic patient do not exclude TB, particularly in a patient who is immunosuppressed.

In primary active TB, radiographic features of pulmonary tuberculosis are nonspecific, sometimes even normal. The chest radiograph typically shows a pneumonialike picture of an infiltrative process in the middle or lower lung regions, often associated with hilar adenopathy and/or atelectasis.

In classic reactivation TB, pulmonary lesions are located in the posterior segment of the right upper lobe, apicoposterior segment of the left upper lobe, and apical segments of the lower lobes. Cavitation is most common; healing of tubercular regions results in the development of a scar with loss of lung parenchymal volume and calcification.

In the presence of HIV or another immunosuppressive disease, lesions are often atypical. Up to 20% of patients who are HIV positive with active disease have normal chest radiographic findings.

Old, healed TB presents differently, with dense pulmonary nodules found, with or without calcifications, in the hilar or upper lobes. Smaller nodules, with or without fibrotic scars, can be seen in the upper lobes. Nodules and fibrotic lesions are well demarcated, have sharp margins, and are dense. Persons with nodular or fibrotic scars with positive chest radiographic findings and positive PPD results should be treated as latent carriers. Calcified nodular lesions (granulomas) or apical pleural thickening has a lower risk of conversion.

In disseminated/miliary tuberculosis, the chest radiograph commonly shows a miliary pattern, with 2-mm nodules that are histologically granulomas disseminated like millet seeds throughout the lung; however, chest radiographic patterns can vary and can include upper lobe infiltrates with or without cavitation.

In pleural tuberculosis, the pleural space can be involved in 2 ways: a hypersensitivity response with pleuritic pain and fever, or an empyema that can be seen on chest radiograph with associated pleural effusions.

See the following articles for more information:

• Imaging of CNS Tuberculosis
• Imaging of Primary Pulmonary Tuberculosis
• Imaging of Gastrointestinal Tuberculosis
• Imaging of Genitourinary Tuberculosis

CT scanning

CT scanning of the chest may help to better define abnormalities in patients with vague findings on chest radiography.

If vertebral involvement (Pott disease) or brain involvement is suspected in a patient, it is important to consider that a delay in treatment could have severe repercussions for the patient (compression of the spinal cord and/or paraplegia); further evaluation is necessary with computed tomography (CT) scanning or magnetic resonance imaging (MRI).

Technetium scanning

Technetium-99m (^99m Tc) methoxy isobutyl isonitrile single-photon emission CT (SPECT) scanning for solitary pulmonary nodules yields a high predictive value for distinguishing TB from malignancy. Therefore, it has the potential to serve as a low-cost alternative when positron emission tomography (PET) scanning is not available, especially in endemic areas.^[35]

The primary screening for TB infection (active or latent) is the tuberculin skin test with purified protein derivative (PPD).

The mechanism of tuberculin skin testing is based on the fact that latent TB infection induces a strong cell-mediated immune response by measuring the delayed-type hypersensitivity response to intradermal inoculation of tuberculin PPD.

The PPD test is given in an intradermal injection of 5 units of purified protein derivative, preferably with a 26-, 27-, or 30-gauge needle. These delayed-type hypersensitivity tests should be read between 48 and 72 hours after administration.

A negative response in immunologically intact individuals measures less than 5 mm.

Population-based criteria for PPD positivity are as follows:

• For patients who are HIV positive, have abnormal chest radiographic findings, have significant immunosuppression, or have had recent contact with persons with active TB, the cutoff is 5 mm or more induration.
• For patients who are intravenous drug users, residents of nursing homes, prisoners, impoverished persons, or members of minority groups, the cutoff is 10 mm or more induration.
• For patients who are young and in good health, the cutoff is 15 mm or more induration.

Reactions in patients who have received the bacilli Calmette-Guérin (BCG) vaccine should be interpreted the same as above, regardless of BCG history, according to CDC guidelines.

An in vitro blood test based on interferon-gamma release assay (IGRA) with antigens specific for M tuberculosis can also be used to screen for latent TB infection and offers certain advantages over tuberculin skin testing.^{[36, 37]} Currently available tests include QuantiFERON-TB Gold In-Tube (QFT-GIT), an enzyme-linked immunosorbent assay or ELISA based on ESAT-6, CFP-10, and TB 7.7 antigens and T-SPOT.TB, an enzyme-linked immunospot (ELISpot) assay based on ESAT-6 and CFP-10 antigens. Both tests measure in vitro T-cell interferon (IFN)-gamma in response to antigens highly specific for M tuberculosis and absent from the BCG vaccine and M avium.^[38]

Overall, sensitivity and specificity of IGRA are comparable to those of tuberculin skin testing; however, unlike tuberculin skin testing, a second encounter for reading is unnecessary. Results are reported as positive, negative, or indeterminate. Patients with an indeterminate result may have evidence of immunosuppression and may be nonreactive on skin testing.^[39]

Neither tuberculin skin testing nor IGRA testing is sufficiently sensitive to rule out TB infection.^[40] Approximately 20% of patients with active TB, particularly those with advanced disease, may have normal PPD test results.

Limited data exist on the sensitivity of TST and IGRA tests in some situations; caution is recommended on the interpretation of these tests in infants and patients with immunosuppressive conditions.^[38]

A systematic review of QuantiFERON-TB Gold (QFT-G)/Gold in-Tube (QFT-G-IT) and T-SPOT.TB by Chang and Leung concluded that QFT-G had the highest positive likelihood ratio (48.1) for latent TB infection and T-SPOT.TB had the best negative likelihood ratio (0.10). A negative T-SPOT.TB result in middle-aged and older patients makes active TB very unlikely.^[41]

Results from a study by Leung et al indicated that tuberculin skin testing was not predictive of the subsequent development of active TB.^[42] The authors followed 308 males with increased risk of TB due to a diagnosis of silicosis. A positive T-SPOT.TB finding was associated with a relative risk of 4.5 for subsequent TB in the group overall and a relative risk of 8.5 among the men who did not receive preventive treatment for latent TB. CFP-10 spot count was more predictive than the ESAT-6 spot count.

In a separate study by Diel et al, all subjects who developed active tuberculosis within 4 years after exposure to a smear-positive index case had positive results using QuantiFERON-TB Gold in-tube.^[43]

In a study of kidney-transplant recipients, isoniazid therapy was given to all patients with a significant TST reaction or risk factors for TB infection. ELISPOT assay was performed on all patients. No patients who were treated with isoniazid developed active TB. Among 71 patients with positive ELISPOT who did not receive isoniazid, 4 (6%) subsequently developed active TB after kidney transplantation.^[44]

A systematic review of QuantiFERON-TB Gold (QFT-G)/Gold in-Tube (QFT-G-IT) and T-SPOT.TB by Chang and Leung concluded that, at a 90% certainty threshold, latent TB infection is best diagnosed with QFT-G/QFT-G-IT and best excluded with T-SPOT.TB. Neither test can diagnose TB disease, but T-SPOT.TB can exclude it in middle-aged and older patients.^[41]

Advantages to IGRA compared with PPD include the following:

• One patient visit
• Ex vivo tests
• No booster effect
• Independent of BCG vaccination

Disadvantages of IGRA include the following:

• High cost
• More laboratory resources required
• Complicated process of lymphocyte separation
• Lack of prospective studies

Jafari et al found that an M tuberculosis –specific ELISpot assay can be used to differentiate TB cases with sputum smear negative for acid-fast bacteria (AFB) from latent TB infection. In a prospective study of 347 patients suspected of having active TB who were unable to produce sputum or who had AFB-negative sputum smears, ELISpot testing of bronchoalveolar lavage fluid displayed a sensitivity and specificity of 91% and 80%, respectively, for the diagnosis of active pulmonary TB.^[45]

Other rapid tests are also available, such as BACTEC-460 (Becton-Dickinson), ligase chain reaction; and luciferase reporter assay (within 48 h) (Franklin Lakes). These tests have been developed for rapid drug-susceptibilities testing, which can be available within 10 days.

Drug resistance tests such as the FASTPlaque TB-RIF for rifampin resistance can be used after growth in semiautomated liquid cultures such as BACTEC-460; rifampin resistance can be used as a surrogate marker for isoniazid resistance.

Urinalysis and urine culture can be obtained for patients with genitourinary complaints. Patients are often asymptomatic; however, significant pyuria and/or hematuria with no routine bacterial organisms should prompt urine culture for acid-fast bacilli.

All patients who are diagnosed with active tuberculosis (TB) and who are not known to be HIV positive should be considered for HIV testing.