Diagnosis of an Infection Flashcards
What general symptoms are seen with infections?
- Pyrexia
- General malaise
- Rash
- Headache, abdominal, or respiratory symptoms
- Septic shock
What may the pattern of pyrexia be specific to?
Different infective causes
What conditions, other than infections, can cause pyrexia?
- Malignancy
- Drugs
- Other diseases
What is meant by general malaise?
Flu-like symptoms
What does general malaise result from?
Cytokines
When is septic shock produced?
Severe infection
What may the site of the actual infection show?
- Local pain
- Discharge
- Coughs
- Sputum production
- Diarrhoea
How can infectious agents be categorised?
- Toxin mediated
- Acute pyogenic
- Sub-acute
- Chronic granulomatous
What is it important to consider when looking at infection to give a better understanding of the disease?
The exposure to the likely source
What are the categories potential exposures to the source?
- Endogenous
- Exogenous
What should be do when it is suspected an individual has an infection?
- History taken
- Clinical examination
- Samples for microbiological testing
What is the purpose of the history and clinical exam when an individual is suspected to have an infection?
So a differential diagnosis can be produced
What types of specimen can be taken for microbiological investigation?
- Blood
- Pus
- Lavage
What should happen once a specimen has been taken?
It should be cultured
Why should a specimen be cultured when taken for clinical examination?
To increase the number of microbes grown for identification
What can cultures be used to do?
Limit the growth of certain pathogens to reduce any contaminants in the sample
How may a clinical sample for investigation get contaminated?
Poor aseptic technique obtaining the sample
What potential problem could arise when trying to culture a sample for microbiological investigation of an infection?
Many organisms are obligate anaerobes, which will be killed in exposure to an atmosphere with oxygen
What is required for viruses to grow?
A cell culture
What should happen following the obtaining of a clinical specimen?
The sample must be analysed
What are the categories of mechanisms for analysing a clinical specimen?
- Microscopy
- Molecular techniques
- Serology
How can the sample be analysed microscopically?
Via various staning or flouresence
What stain is used for bacteria?
A gram stain
For what bacteria can a gram stain not be used?
Mycobacterium
What stain is require to examine mycobacterium microscopically?
ZN stain
What samples can be examined microscopically directly (with no stain)?
Stool samples
What can be indentified when examining a stool sample directly?
Parasites
What can be used for viral detection?
Electron microscopy
What can be used to diagnose RSV?
Flouresence
What is the main molecular technique used in microbiological examination?
Nucleic acid amplification testing (NAAT)
What is NAAT used to identify?
Both bacteria and viruses
How does NAAT work?
It amplifies target DNA or RNA, until sufficient quantities have been cultured, and machines can then test the nucleic acid and identify the species
What is NAAT very useful for?
Organisms that are slow growing or dangerous to grow
What is serology?
Testing for the blood serum for an immune response against a specific pathogen
How is a diagnosis made using serology?
By detecting a rise in antibody levels, or the presence of an IgM antibody
What techniques of serology are used?
- Agglutination
- Enzyme immunoassay
- Complement fixation
What does agglutination involve?
The clumping together of antibodies to produce a large group, which can then be identified to identify the bacteria involved
What happens in enzyme immunoassay?
Antibodies specific for the IgG antibodies for certain reactions are on a solid support, and will produce a response if they bind to the specific IgG antibody for the certain microbe
What are antibody levels often quoted as?
Titres
What are titres?
The reciprocal of the last dilution at which Ag-Ab complex could be detected
What should be done if the laboratory findings do not match the clinical findings?
The laboratory results should be questioned for a potential false positive
What is it important is done for the final diagnosis?
The microbiology is matched to the clinical findings
What can be done following identification for some bacteria and fungi?
Check the antibiotic susceptibility
How can bacteria be classified?
With regards to antibiotic susceptibility
- Susceptible
- Moderately resistant
- Resistant
What is meant by a susceptible bcateria?
One where a normal dose of antibiotic results in cure
What is meant by a moderately resistant antibiotic?
One where a large dose of antibiotic results in cure
What is meant by resistant antibiotic?
One where antibiotics are likely to fail
What is the main technique for determining antibiotic susceptibility?
Grow the culture in medium, and placing antibiotic impregnanted disks into the medium and measuring the diameter on inhibition on the plate
What does testing for antibiotic susceptibility provide?
An estimate into how well the patient will react to therapy
What is the limitation of testing for antibiotic susceptibilty?
The clinical response will also depend on host factors
Other than using discs, how can antibiotic susceptibility be checked?
Using broth dilution tests
What do broth dilution tests provide?
- A minimum inhibitory concentration (MIC)
- A minimum bactericidal concentration (MBC)
What is the minimum inhibitory concentration?
The minimum concentration of antibiotic that can be given to inhibit growth
What is the minimum bactericidal concentration?
The minimum concentration of an antibiotic that can be given to kill the bacteria
Label this picture of a broth dilution test
- A - Minimum Inhibitory Concentration
- B - Minimum Bactericidal Concentration
- C - Growth on subculture
What are the advantages of culture dependant identification?
- Can test other properties
- Sensitive and specific
What other properties can be tested for using culture dependant identification?
- Sensitivity
- Virulence
- Phenotype
What are the disadvantages of culture dependant identification?
- Slow
- Organism must be alive
- Low yield post-antibody response
- Method often highly organism specific
What are the advantages of culture independant identification?
- Rapid
- Organism can be alive or dead
- Suitable after antibody response
- Sensitive
What are the disadvantages of culture indepedant identification?
- Limited testing of other properties
- Potential for contamination
- Same methods apply to all organisms
