Diagnosis of Tuberculosis

Clinical and anamnestic stage and risk groups

Diagnostic workup always begins with a comprehensive history taking. The main goal is to determine the degree of risk for the disease. The physician investigates the presence of epidemiological contact with a tuberculosis patient: its duration, frequency, presence of bacterial excretion, and data on the drug susceptibility of the pathogen are clarified. Infants and young children are most often infected by adolescents or adults, and the incidence in such foci is several times higher than the population rate.

Vaccination status (BCG, BCG-M), results of previous treatment by a phthisiatrician, presence of comorbidities, and long-term medication use are mandatory assessments. An important condition is chest X-ray for all family members to detect a possible hidden focus of infection. Objective examination includes anthropometry, palpation of all peripheral lymph node groups (searching for specific lymphadenopathy), as well as standard percussion and auscultation.

Immunodiagnostics: Mantoux test and Diaskintest

Assessment of specific reactivity is based on two complementary intradermal tests.

Mantoux test is used for mass screening (annually for children aged 1 to 18 years, preferably in the same season), early detection of infection, and selection for BCG revaccination. Purified tuberculin is used—a heat-inactivated protein filtrate of a culture mixture of M. tuberculosis and M. bovis. A standard dose (2 TU in 0.1 ml) is injected intradermally on the inner surface of the forearm.

Diaskintest (test with recombinant tuberculosis allergen) contains two proteins specific exclusively to actively multiplying mycobacteria. Unlike tuberculin, it does not cross-react with the BCG vaccine strain. The test is characterized by high sensitivity (98–100%) and specificity (90–100%) in active disease, low allergenicity, and rare development of systemic reactions (fever, headache).

Interpretation of skin test results

Assessment of the local reaction is performed strictly after 72 hours. Only the transverse diameter of the infiltrate (papule) is measured using a transparent millimeter ruler.

Criteria for evaluating the Mantoux test:

• Negative: 0–1 mm.
• Doubtful: 2–4 mm (or presence of hyperemia only without induration).
• Positive: 5–16 mm.
• Hyperergic: 17 mm and more, or reactions of any size with vesiculonecrotic changes and lymphangitis.

For Diaskintest, a papule of absolutely any size is considered positive, and 15 mm or more (or presence of vesicles) is considered hyperergic.

A key concept in diagnostics is tuberculin skin test conversion (the first appearance of a positive reaction after negative ones). It is important for the clinician to differentiate infectious allergy (true infection with a wild strain) from post-vaccination allergy (normal reaction to BCG). Sharply positive and hyperergic reactions are always interpreted as an absolute sign of infection.

Laboratory and radiation diagnostics

A pathognomonic (absolutely reliable) sign of tuberculosis is bacteriological confirmation—direct detection of Mycobacterium tuberculosis in sputum, bronchial or gastric lavage water, urine, or feces. Direct microscopy, culture, flotation methods, and biological assay are used. However, sputum microscopy detects no more than 50% of newly diagnosed cases and does not differentiate the pathogen from non-tuberculous mycobacteria. In young children, bacterial excretion is rare, so microbiological verification is less informative in them.

The clinical blood test reveals non-specific signs of inflammation: a moderate increase in ESR, slight leukocytosis with a left shift, lymphopenia, and eosinophilia.

To confirm pulmonary forms, determine the localization and extent of the process, a two-projection chest X-ray, chest CT, or tomosynthesis is mandatory. Ultrasound, bronchoscopy, or biopsy are prescribed according to indications.

Differential diagnosis and indications for consultation

Patients with abnormal reactions to immune tests (newly positive or increasing Mantoux, positive Diaskintest), abnormal findings on the radiograph, or clinical symptoms suspicious for tuberculosis are referred to a phthisiatrician.

Isolated enlargement of intrathoracic lymph nodes is differentiated from tuberculous bronchitis, sarcoidosis, and non-Hodgkin lymphomas. When differentiating with sarcoidosis, it is important to remember: in active sarcoidosis, the Mantoux test and QuantiFERON test (IGRA assay for Mycobacterium tuberculosis complex antigens) are usually negative (in 80–85% of cases). A positive reaction (papule from 5 mm) requires thorough exclusion of tuberculosis infection.

Class and exam questions

What else to study on this topic

• Differential diagnosis of infectious and post-vaccination allergy
• Features of microbiological diagnostics in young children
• Differential diagnosis of tuberculosis and sarcoidosis
• Indications for computed tomography and chest tomosynthesis
• Instrumental methods in extrapulmonary forms of tuberculosis

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.