Diagnostic methods for infectious diseases

Clinical and epidemiological stage

The basis for establishing a provisional diagnosis is a comprehensive assessment of clinical and epidemiological data. The physician analyzes anamnestic, epidemiological, and physical findings.

Epidemiological history helps establish the potential source and routes of transmission. The interview determines:

• Facts of business trips or stays in endemic areas.
• Consumption of potentially hazardous foods (e.g., unwashed fruit in intestinal infections).
• Contacts with infection reservoirs, as many severe diseases (anthrax, leptospirosis) are zoonotic infections.

Clinical presentation is evaluated through the lens of leading syndromes. Using acute shigellosis (colitic variant) as an example, the following are distinguished:

• Intoxication syndrome: general weakness, headache, chills, and fever.
• Distal spastic colitis syndrome: lower abdominal pain, painful false urges to defecate (tenesmus), frequent stools with pathological impurities (mucus and blood).
• Objective findings: abdominal palpation reveals a painful, dense, and spastic sigmoid colon.

In other infections, such as leptospirosis, the clinical search aims to identify hemorrhagic, meningeal, and encephalitic syndromes, as well as signs of acute renal or hepatic failure.

Etiological confirmation and rapid diagnostics

After forming a provisional diagnosis, laboratory diagnostics are prescribed.

Bacteriological method (culturing the test material on selective nutrient media) is the gold standard of diagnostics. Depending on the portal of entry of the infection and the routes of dissemination (lymphogenous, hematogenous, parenchymatous diffusion), the test material may be stool, blood, or other fluids.

Critically important condition: the prescription of etiotropic therapy (e.g., antibacterial drugs) must occur strictly after the collection of biological material for bacteriological examination. Otherwise, the growth of the pathogen on media will be suppressed, making it impossible to confirm the diagnosis.

For early (rapid) diagnostics, methods aimed at the rapid detection of pathogen antigens are used. A prime example is the coagglutination test. When shigellosis is suspected, this method allows bacterial antigens to be detected in saliva, blood, or coprofiltrate in the shortest possible time. The arsenal of infectious disease specialists also includes the bacterioscopic method—visualizing the pathogen under a microscope (used, in particular, in leptospirosis).

Serological, molecular, and biological methods

If the pathogen itself cannot be isolated, indirect or high-tech direct methods are used:

1. Serological diagnostics. Aims to detect specific antibodies in the patient's blood serum. A classic example is the IHA (indirect hemagglutination assay) using specific diagnostic agents (e.g., dysenteric). A feature of the method is that it becomes informative only from the end of the first week of the disease, when the immune system produces a sufficient amount of antibodies.
2. Molecular genetic methods. Allow the detection of the pathogen's genetic material. These are highly sensitive tests used in the diagnosis of dangerous infections such as anthrax (Anthrax).
3. Immunological and allergological methods. Used to assess the specific reactivity of the body to the invasion of the pathogen. Allergological tests are particularly relevant in the diagnosis of anthrax.
4. Biological assay. A specific method in which susceptible laboratory animals are infected with the test material. This allows the accumulation and isolation of a pure culture of the pathogen, which is actively used in the diagnosis of leptospirosis.

Class and exam questions

What else to study on this topic

• Pathogenesis of leptospirosis: mechanisms of development of hemorrhagic and hemolytic-uremic syndromes.
• Clinical classification of anthrax: localized and generalized forms.
• Differential diagnosis of icteric and anicteric variants of leptospirosis.
• Etiotropic therapy of infections: drug selection and duration of courses depending on severity.
• Specific therapy: indications for the use of anti-leptospiral gamma globulin.

Other topics in the "Acute Intestinal Infections" section

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