Viruses mainly infect the human respiratory tract, which can lead to many different clinical manifestations. The most common of these are upper respiratory tract infections (IVP). Children tolerate 6 to 9 IVDP per year, and their number decreases to 2-4 in adolescence and in adults. Lower respiratory tract infections (INDs) are much less common, but are much more expensive. About a third of children suffer from LID in the first year of life. In school-aged children, this proportion is 5-10%, in healthy adults less than 5%, and increases again to about 17% in elderly patients. The proportion of hospital visits in the United States for children and adults with viral PID is very large. The groups at high risk for viral SAR are children; immunocompromised patients; people with chronic diseases (heart, lungs, kidneys, etc.) as well as the elderly; hospital patients and military personnel.

The number of children hospitalized each year in the United States for viral lower respiratory infections has recently been estimated at 200,000 with a specific diagnosis (when the etiology of the disease is established) and an additional 210,000 with a clinical diagnosis. The cost of treatment is estimated at nearly one billion US dollars.

Although respiratory viral infections cause significant morbidity and mortality, laboratory diagnosis is not necessary for all patients and not always. Specific advice is needed to know when laboratory use is warranted.

Many viral infections initially cause symptoms in the airways. Most respiratory viruses enter the body through the lining of the nose, pharynx, trachea, and bronchi. Some viruses (for example, the coronavirus associated with SARS and certain types of adenovirus) can actually enter the gastrointestinal tract and then spread through the blood to the lungs. The most common respiratory viruses are influenza viruses types A and B, respiratory syncytial viruses types A and B, parainfluenza viruses types 1, 2 and 3, human metapneumovirus, adenoviruses (subtypes A to E ), rhinoviruses (many subtypes), human coronaviruses 229E, OS43 and coronaviruses causing severe acute respiratory syndrome (SARS).

Many patients with lower respiratory tract infections who are screened using laboratory diagnostic techniques produce negative results. Some factors leading to negative laboratory results may include unidentifiable agents (for example, a coronavirus causing SARS, a human metapneumovirus), low viral load and the presence of inhibitors in a clinical sample, poor quality collection clinical material and its treatment. Currently, the following laboratory methods are known for respiratory viruses: culture, serology, microscopy (including electron microscopy), indirect and direct immunofluorescence methods, enzyme immunoassay, enzyme activity tests (neuraminidase test) and amplification nucleic acids (eg reaction polymerase chain - PCR).

The cultural method is used as a gold standard, but over the past 10 years it has been gradually replaced by molecular methods. Culture in the near future will also remain an important method for maintaining viral cultures used to analyze genetic and antigenic changes in viral populations (eg, early influenza vaccine) and to discover new unknown viruses (eg, metapneumovirus human). However, laboratories with sufficient skills and equipment for such work are becoming less and less due to the reduced need for such research, therefore the organization of regional centers for such work is necessary.

Serological methods are generally not very informative in case of acute infection. In addition, 10 to 30% of patients with confirmed respiratory viral infection are HIV negative.

Currently, express analysis techniques have been developed rapidly. For over 30 years, antigen tests have been used, including the enzyme-linked immunosorbent assay (ELISA). The immuno-optical analysis and the analysis of neuraminidase activity are much more recent. These methods are still widely used and their main advantages are their low cost and their simplicity. However, they suffer from a lack of sensitivity and specificity, especially in specific populations such as immunocompromised patients or the elderly. Until sensitive and specific techniques become cheaper and easier, antigenic techniques will be widely used.

Molecular biological methods have now become standards for the determination of viral agents. The published data on the sensitivity of such methods are generally close to 100%, in fact, the studies carried out show a significant excess of the sensitivity parameters compared to standard methods (up to 30%). However, there is currently no FDA approved method for diagnosing common respiratory viruses. Most molecular methods are based on PCR. There are also works using NASBA.

The oldest and most used molecular commercial technique is Hexaplex based on multiplex PCR to identify the seven most common respiratory viruses. The results of multiplex PCR show a very close correlation with plaque formation, but PCR is much more sensitive.

Another molecular biology method is real-time PCR. It has fewer steps, less risk of contamination, which is a potential problem for most molecular methods. An additional advantage compared to standard format PCR is also the high speed of analysis. Disadvantages include the high cost of equipment, reagents, difficulty defining gray area values with low viral load, instability during storage of commercial reagents, and the inability to simultaneously identify more than 3- 4 agents.

Currently, academic institutions, commercial and military laboratories are doing a lot of work to develop methods for determining respiratory viruses. This work is stimulated by the threat of bioterrorism. The SARS epidemic and the prospect of influenza pandemics motivate many companies to develop technologies to rapidly identify known and unknown respiratory pathogens. Some of these methods, which are under development, will determine the pathogen in an hour, although it is not known how much the equipment will cost to use such methods. Here are some of these devices:

Although the future of respiratory virus diagnosis seems fairly certain, the following potentially important features of molecular diagnosis should be taken into account: