Sunday, May 9, 2021

Approach for Diagnosis of Infectious diseases

Approach for Diagnosis of Infectious diseases


Step 1: The diagnosis of a microbial infection begins with an assessment of clinical and epidemiologic features, leading to the formulation of a diagnostic hypothesis.

Step 2: Anatomic localization of the infection with the aid of physical and radiologic findings (e g right lower lobe pneumonia, sub-phrenic abscess) is usually included for clinical diagnosis of infectious diseases.

However in some instances, site of infection can’t be localized; then, a multi tasking approach will be required.

Step 3: Having clinical diagnosis based upon above two steps will help to predict a possible etiologic agent or a number of possible etiologic agents based on knowledge of infectious syndromes and their courses.

The specific cause is then established by the application of methods described in next chapters.

A combination of science and art on the part of both the clinician and microbiologist is required

Step 4: The clinician must select the appropriate tests and specimens to be processed and, where appropriate, suggest the suspected etiologic agents to the laboratory.

Step 5: The microbiologist must use the methods that will demonstrate the probable agents & provide evidence of existence of causative etiological agent, and be prepared to explore other possibilities suggested by the clinical situation or by the findings of the laboratory examinations.

Most important: The best results are obtained when communication between the clinician and microbiologist is maximal.

 

The general approaches to laboratory diagnosis vary with different microorganisms and infectious diseases. However, the types of methods are usually some combination of

(A)    The investigations which supports clinical diagnosis of possibilities of infection:

e.g. Total & Differential count of WBCs, Stool / Urine routine & microscopic examination etc.

 

(B) The investigations which gives direct evidence of presence of aetiological organisms in the specimen:

These includes,

1.  Direct Microscopic examination: Gram stain, Z N stain, KOH examination for fungus etc

2. Culture techniques: For isolation & confirmative identification of aetiological agent. This also helps to perform antimicrobial sensitivity testing of the pathogen, which in turn helps to select treatment for patient.

3.  Antigen detection: By immunological techniques e.g. Immuno-chromatography

4.     Nucleic Acid Detection: By various Nucleic Acid Amplification Techniques (NAAT) e.g. Polymerase Chain Reaction (PCR)

 

(C)   The investigations which indirectly suggest possibility of infection with particular organisms

This includes various immunological tests those indicate immune response of the body against particular pathogens

1.     Detection of antibodies in patient’s serum against specific pathogen

2.     Detection of cell mediated immune response in patient against specific pathogen by in vivo (Skin test) or in vitro methods

THE SPECIMEN

-      All diagnostic approaches begin with some kind of specimen collected from the patient.

-    The primary connection between the clinical encounter and the diagnostic laboratory is the specimen submitted for processing. If it is not appropriately chosen and/or collected, no degree of laboratory skill can rectify the error. Failure at the level of specimen collection is the most common reason for failing to establish an etiologic diagnosis, or worse, for suggesting a wrong diagnosis.

-      In the case of bacterial infections, the primary problem lies in distinguishing resident or contaminating normal floral organisms from those causing the infection.

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