Diagnosing Infections

Question 1

As a clinician, what are the steps you should follow to diagnose infection?

Answer 1

1. Make a clinical diagnosis based on: history & examination
2. Confirm with special investigations: imaging, lab tests etc.
3. Don’t request tests blindly!

Question 2

What is the basic idea of Koch’s postulates?

Answer 2

In order to make a specific aetiological diagnosing of an infection, the organism, component or product must be demonstrated and isolated. Finally a serological response must be demonstrated.

Question 3

How can you demonstrate the presence of a given organism?

Answer 3

Organisms, their components and/or products can be demonstrated through microscopy, or by detecting specific components such as antigens, nucleic acids or toxic products etc.

• Microscopy can be either unstained (sweat, phase contrast, darkground) or stained (gram, Ziehl-Neelson etc.)
• via antigen detection, by a process called latex agglutination.
• via nucleic acid detection by the processes of hybridisation or PCR.

Some confirmatory/further tests include:

• biochemical tests, MALDI-TOF or DNA/RNA analysis to confirm identity
• Serotyping or pathotyping to determine the subtype
• Determine antimicrobial susceptibility

Question 4

What is PCR?

Answer 4

In the process of PCR, target DNA is ‘melted’ to give single strands. Primer pairs bind to homologous sequences in that target (one to each strand) and then DNA polymerase is used to synthesis DNA extending from bound primer and incorporating free nucleotides from the solution.

This yields two new strands of DNA; each complementary to and bound to one strand of the target.

This process is repeated 30-40 times and the product is then detected and identified by comparing it to DNA samples of known length.