Investigating Infectious Diseases

Question 1

What are we trying to achieve when making a diagnosis of an infectious disease?

Answer 1

• Identify the infecting organism - diagnostic advice.
• Susceptibility testing - treatment advice.
• Identify organisms that are spreading - infection control.

Question 2

What are the challenges of using microbiology results?

Answer 2

• The speed of progression of infection is mich faster than the time taken to generate results (infection is fast and we are too slow, e.g. necrotising fasciitis).
• Laboratory data can be complex.
  
  • S. oralis and S. intermedius are closely related but cause very different diseases.
• The infection specialist can help.

Question 3

Give examples of specimens where you would only expect a single pathogen.

Answer 3

• CSF in meningitis
• Genital specimens when STI are expected
• Throat swab (except ?diphtheria)
• Infection control screening
• Unusual infections e.g. pertussus
• MTB detection

Question 4

Give examples of specimens where you would expect to find multiple pathogens.

Answer 4

• Urine
• Faeces
• Abscess pus
• LRTI samples

Question 5

What are the pros and cons of smear diagnosis?

Answer 5

• Rapid
• Simple to perform
• Cheap
• Not very sensitive
• Not very specific
• Requires considerable expertise

Question 6

What are the pros and cons of culture diagnosis?

Answer 6

• More sensitive than smear
• Allows susceptibility testing
• Allows rapid presumptive diagnosis
• Allows detailed identification
• Rendered negative by ABx

Question 7

What is measured when making a serological diagnosis?

Answer 7

• Detect high IgG concentration
• Detect rising of falling titres
• Detect IgM / IgA
• Measure avidity of binding
• Detect antigen
• Eamples of serological techniques
  
  • Agglutination
  • Precipitation
  • Complement fixation
  • Radioimmunoassay

Question 8

What is sensitivity?

Answer 8

• The ability of a test to detect all of the true positives.
• Equal to the number of positives obtained divided by the total number of positives.

Question 9

What is specificity?

Answer 9

• The ability of a test to indentify all of the true negatives.
• Equal to the number of negatives obtained divided by the number of true negatives.

Question 10

What are the normal flora of the nasopharynx?

Answer 10

• Streptococci
• Haemophilus
• Neisseria
• Mixed anaerobes
• Candida
• Actinomyces

Question 11

What are the normal flora of the skin?

Answer 11

• Staphylococci
• Streptococci
• Corynebacteria
• Proprionibacteria
• Yeasts

Question 12

What are the normal flora of the upper bowel?

Answer 12

• Enterobacteriaceae
• Enterococci
• Candida

Question 13

What are the normal flora of the lower bowel?

Answer 13

• Bacteroides
• Bifidobacteria
• Clostridium
• Peptostreptococci

Question 14

What are the normal flora of the vagina?

Answer 14

• Lactobacilli
• Streptococci
• Corynebacteria
• Candida
• Actinomyces
• Mycoplasma hominis

Question 15

What are the drawbacks of agar based respiratory diagnosis?

Answer 15

• Legionella
  
  • ​Grows slowly and requires secialist medium.
• Mycoplasma pneumoniae
  
  • ​Requires specialist medium and 14 days to grow.
• Chlamydia psittaci, Chlamydophyla pneumoniae
  
  • ​Obligate intracellular pathogen.