Clinical Specimens and Rapid Diagnostics
The initial and most critical step in diagnostics is the proper collection of clinical material. When viral respiratory tract infections are suspected, respiratory secretions are sent for analysis: washes and impression smears from the nose and throat, as well as nasopharyngeal mucus.
To obtain results as quickly as possible in modern clinical practice, rapid diagnostic methods are used. Their main goal is the prompt detection of pathogen traces in the patient's body:
- Detection of viral antigens. Performed directly in infected cells of the respiratory tract using direct or indirect immunofluorescence assay (DFA) and enzyme-linked immunosorbent assay (ELISA).
- Detection of genetic material. Performed using polymerase chain reaction (PCR). This method is critically important and especially relevant when searching for fastidious viruses that are difficult or impossible to grow using standard laboratory methods.
Virological Method (Cultivation)
The classical virological method involves the isolation of the pathogen. Cell cultures serve as the growth medium, which are inoculated with the patient's respiratory secretions.
After inoculating the material, the process is divided into two consecutive stages:
1. Virus Indication This is the stage of primary detection indicating the presence of any virus in the culture. Successful replication of the pathogen is signaled by the following phenomena:
- Appearance of a cytopathic effect (CPE) — visible cellular changes or cell death.
- Positive hemagglutination assay (HA) and hemadsorption reactions (characteristic of viruses with specific hemagglutinating properties).
- Formation of specific intracellular inclusions. The location of the inclusions can suggest the virus type: for example, intranuclear inclusions are typical for adenoviruses, while perinuclear cytoplasmic inclusions are typical for reoviruses.
- Use of the plaque assay and the color test.
2. Virus Identification Once the presence of a virus is proven, its species must be determined. Identification is carried out by studying the antigenic structure of the pathogen. A wide range of serological reactions is used for this purpose: complement fixation test (CFT), passive hemagglutination inhibition test, ELISA, hemagglutination inhibition test (HI), and neutralization test (NT).
Serological Method
Serological diagnostics is based on evaluating the humoral immune response to the pathogen. The main principle of the method is testing paired serum samples from the patient.
Key features of the serological approach:
- Timing of collection: the first serum sample is collected at the onset of the disease, and the second is collected 10–14 days later.
- Diagnostic criterion: the diagnosis is considered confirmed only if there is a 4-fold or greater rise in specific antibody titers.
- Reactions used: neutralization test (NT), complement fixation test (CFT), passive hemagglutination assay (PHA), and hemagglutination inhibition test (HI).
It is important to note the clinical significance of this method. Since the average duration of an uncomplicated ARVI is only 5–7 days, the serological method takes longer than the disease itself (due to the time required to mount an antibody response). Therefore, it is primarily used for retrospective diagnostics (confirming a past infection) and detailed population-level epidemiological analysis.