Direct and indirect methods for the diagnosis of bacterial infection – bacteremia

All methods for the diagnosis of bacterial infections can be divided into two main groups – direct and indirect. Straight lines make it possible to isolate a pathogen or one of the products of its vital activity. An example is the detection of microorganisms in the blood, cerebrospinal fluid, urine by sowing or staining using the Gram method.

In addition, bacteria can be identified by immunofluorescence and by detecting an antigen in blood, cerebrospinal fluid, or urine using latex agglutination or counter current immunoelectrophoresis (PIEF). The last two tests are more sensitive than the Gram method or microscopic examination. The latest achievement in this area is the enzyme-linked immunosorbtion assay (ELISA) (PHIS), which is even more sensitive than PIEF.

A detailed discussion of these methods is not part of our task. Moreover, progress in this area is so rapid that the data cited may very quickly become outdated.

Indirect methods allow the diagnosis of bacterial infections by determining the body’s response to an infection. By their nature, these methods are less specific than the direct tests described above. However, their theoretical advantage is that neither the doctor nor the laboratory technician need to know which probable pathogen caused the disease.

In contrast, methods such as PIEF and SFIS are aimed at identifying a specific pathogen suspected by the doctor and are not able to identify rare or unsuspected microorganisms. Indirect methods such as determining the ESR or the total number of leukocytes in the blood, allow the doctor to think about an infectious disease, even if he is completely unfamiliar with its pathogen. However, the two main causative agents of fever without local symptoms in previously healthy children – S. pneumoniae and H. influenzae type B – are detectable using PIEF and PSIA.

Another advantage of indirect methods is that they are cheaper, more accessible, and doctors are more familiar with them. However, as technology improves, the situation will change. Obviously, the introduction of automation, computerization and miniaturization will make methods such as the PIEF and the FISS available for widespread use in polyclinic laboratories.

But until these laboratory methods become publicly available, physicians should be able to select groups of patients with the highest statistical risk of developing bacteremia or serious infectious diseases, despite the fact that while we cannot always reliably determine whether a particular patient is infected. Therefore, while widely accepted indirect clinical laboratory tests retain their value.
Source: MedUniver

local_offerevent_note February 9, 2019

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