Differential diagnosis of pulmonary tuberculosis

Criteria for distinction from secondary tuberculosis

The main task when identifying disseminated processes in lung tissue is the differential diagnosis between hematogenous tuberculosis (with predominant pulmonary involvement) and secondary tuberculosis. In the hematogenous form, the formation of tubercles of various calibers directly in the lung tissue predominates, whereas in other organs, granulomas are absent or solitary. The course of the process varies from acute to chronic, and based on the size of the foci, miliary and large-focal variants are distinguished.

To reliably distinguish this form from secondary tuberculosis, it is necessary to rely on four basic pathomorphological signs:

• Presence of an extrapulmonary primary focus. Hematogenous dissemination implies the presence of a source from which mycobacteria entered the bloodstream.
• Symmetry of changes. In hematogenous spread, both lungs are affected.
• Absence of layered "tiering". Secondary tuberculosis is characterized by the presence of foci of different ages arranged in layers (tiers). In the hematogenous variant, there is no such morphological regularity.
• Perifocal fibrosis. Hematogenous forms are prone to the progressive development of connective tissue around the foci, eventually leading to the formation of characteristic "punched-out" cavities.

Impact of interstitial diseases on diagnosis

Differential diagnosis is significantly complicated if the patient has concomitant pulmonary interstitium pathologies. Tuberculosis is one of the complications of usual interstitial pneumonia (UIP) and idiopathic pulmonary fibrosis (IPF).

In these diseases, the parenchyma is already structurally altered: HRCT reveals traction bronchiectasis and basal subpleural "honeycomb lung". If tuberculosis develops against this background, its visual characteristics are severely distorted by background pneumofibrosis. The radiological picture becomes completely atypical. In such cases, the diagnosis cannot be made solely by CT — confirming specific inflammation requires a biopsy of the identified consolidation zone.

In addition, when differentiating CT patterns, the physician must rule out other diseases, such as nonspecific interstitial pneumonia (NSIP) with a prominent "ground-glass opacity" symptom or smoking-related pathologies (desquamative interstitial pneumonia).

Differentiation within hematogenous forms

According to the classification of A.I. Strukov and V.V. Serov, hematogenous tuberculosis is heterogeneous. Diagnosis is carried out between three main groups:

1. Generalized hematogenous process. Develops in immunocompromised individuals.
2. With predominant pulmonary involvement. Acute or chronic miliary and large-focal tuberculosis.
3. With predominant extrapulmonary involvement. Involves bones, joints, skin, and the urogenital system, going through focal and destructive stages of development in the organ. Notably, intestinal tuberculosis can mimic colitis, requiring differential diagnosis with ulcerative colitis, Crohn's disease, and infections (including antibiotic-associated conditions caused by Clostridioides difficile and dysentery).

Features of generalized tuberculosis

Generalized forms are characterized by extremely high mortality and a severe clinical course. The pathomorphological picture here is specific. Due to pronounced immunodeficiency and body anergy, the productive reaction is minimal or absent — typical tuberculous granulomas do not form.

• Hyperacute tuberculous sepsis (obsolete term — Landouzy typhobacillosis). At the microscopic level (with hematoxylin and eosin staining), multiple foci of pronounced necrosis without a productive reaction are visible in the liver and other organs. Alternating-necrotic changes predominate.
• Acute generalized miliary tuberculosis. Manifests as the eruption of small productive tubercles in organs. A specific feature is the frequent addition of tuberculous meningitis.
• Acute generalized large-focal tuberculosis. Rare, predominantly in severely cachectic and debilitated patients.

Class and exam questions

What else to study on this topic

• Morphology of chronic hematogenously disseminated pulmonary tuberculosis.
• Stages of development of extrapulmonary lesions (focal and destructive).
• Pathogenesis of tuberculous meningitis in miliary forms.
• HRCT signs of nonspecific interstitial pneumonia (NSIP) and UIP.
• Differential diagnosis of ulcerative colitis, infections, and intestinal tuberculosis.

Other topics in the "Additional Topics" section

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