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Methods of Laboratory Diagnostics

For medical students2 min readUpdated 2026-10-10

Laboratory diagnostics in microbiology relies on a set of approaches aimed at identifying the causative agent and selecting appropriate therapy. For opportunistic infections, the bacteriological method plays the leading role, while other approaches perform indicative, auxiliary, or highly specialized functions.

Gold StandardBacteriological culture allows identification of the pathogen down to the strain level.
IndicatorMicroscopy has low specificity due to the similar morphology of various pathogens.
Low ValueSerology is uninformative due to slow accumulation and low antibody titers.
Narrow ProfileThe allergological method is used in chronic pathologies with an allergic component.

Bacteriological Method (Primary)

This approach is recognized as the "gold standard" in the diagnosis of infections caused by opportunistic microflora. It provides the clinician with the most complete set of data necessary for prescribing targeted antimicrobial therapy.

Key capabilities of the method:

Microscopic Method (Indicative)

Despite its rapid turnaround time, microscopy is strictly indicative. The main limitation of the method is its low specificity. This is because many opportunistic microorganisms share extremely similar morphology (appearance), making an accurate diagnosis impossible based solely on viewing through the microscope objective.

Tasks of microscopy:

  1. Initial confirmation of the mere presence of bacterial cells in the clinical specimen.
  2. Assessment of basic parameters: microbial shape and tinctorial properties (staining characteristics, which directly depend on cell wall thickness, such as in Gram staining).

Serological Method (Low Informational Value)

In purulent-inflammatory processes caused by opportunistic flora, serological diagnostics is significantly inferior in informational value compared to studies used for classical systemic infections.

Reasons for low efficacy:

Allergological Method (Specialized)

This is a highly specialized tool used strictly according to specific indications. The main goal is to detect hypersensitivity of the body to a specific microbe when it is suspected to play a major pathogenetic role.

Indications for use: Chronic diseases characterized by a clearly traceable allergic component. This includes recurrent pathologies of the respiratory tract, gastrointestinal tract, kidneys, joints, and skin.

Technique: Based on skin-allergy tests. Standardized allergen preparations derived from various pathogens are administered to the patient, such as:

Frequently asked questions

What stages does a standard bacteriological study in the laboratory include?

The standard bacteriological study includes consecutive stages:

  • Primary inoculation — applying clinical material to a set of nutrient media.
  • Growth assessment — quantitative characterization of grown colonies: sparse, moderate, or abundant growth.
  • Isolation of a pure culture.
  • Pathogen identification.
What methods exist for rapid identification of microorganisms?

Automated systems, molecular-genetic, and physicochemical methods are used for rapid identification of microorganisms.

  • Automated systems (such as Bactec, MB/BacT) — use fluorimetric registration of metabolism (CO₂ accumulation or O₂ consumption).
  • PCR — molecular method for subsequent precise identification.
  • Gas-liquid chromatography (GLC) — detection of specific volatile metabolites (e.g., propionic or butyric acid).
  • Cultural rapid tests — assessment of specific growth (e.g., at 43 °C in litmus milk).
  • Direct DFA — direct fluorescent antibody test for rapid antigen detection.
How is microbial antibiotic susceptibility assessed using the disk-diffusion method?

The disk-diffusion method is a qualitative study to determine whether a microbe is susceptible or resistant.

  • Preparation and inoculation — an inoculum at a concentration of 1.5×10⁸ CFU/mL according to the McFarland standard is plated onto solid nutrient agar.
  • Application of the agent — special antibiotic disks are placed on the surface of the medium.
  • Incubation — cultures are incubated, and results are read after 24 hours.
  • Evaluation of results — inhibition zones around the disks are measured to evaluate the presence or absence of these zones.
What nutrient media are used for primary inoculation of clinical material?

The choice of nutrient media for primary inoculation depends on the suspected pathogen and the goals of the study.

  • Blood agar (including sheep blood agar) — solid medium for isolating pure culture and assessing hemolysis.
  • Chocolate agar — used for cultivating fastidious bacteria (e.g., Haemophilus species).
  • Thioglycolate / Kitt-Tarozzi broth — liquid enrichment medium for primary inoculation of anaerobes.
  • Sugar agar — used for stab culture inoculation of anaerobes.
  • Selective/differential media (Ploskirev agar, bismuth sulfite agar) — used for isolating enteric pathogens.
What is the gold standard of diagnosis in this case?

The bacteriological method is the gold standard of diagnosis because it allows precise determination of the microbial species, quantity, and drug susceptibility.

What is the main drawback of the microscopic method?

Its main drawback is low specificity. Many opportunistic bacteria look identical under the microscope, so the method only provides indicative information about the presence and shape of the pathogen.

Why is serology rarely used for infections caused by opportunistic flora?

Due to the slow accumulation of antibodies and their comparatively low titers in such purulent-inflammatory diseases. Unlike the diagnosis of classic infections, serology here has low informational value.

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