Infection Tests Flashcards
Why are infection tests performed?
What happens if they are done well / poorly?
they are vital to the optimal treatment and prevention of infection
if they are done well, they can save lives, time and money
if they are done poorly, they can result in loss of life
What types of symptoms mentioned in a history are key to making a diagnosis of an infection?
System specific symptoms:
- cough - respiratory
- neck stiffness - CNS
- bone pain - orthopaedic
- skin pain / redness - skin/soft tissue
- dysuria - urinary
Non specific symptoms:
- help to support a diagnosis of infection
- fever
- shaking episodes / chills
- sweating / night sweats
- feeling muddled / confused
When symptoms suggest a diagnosis of infection, what other information can help identify potential pathogens?
Travel:
- inside UK?
- outside UK? Need details
Animal contact:
- which animal (s) / nature of contact
Occupation:
- e.g. farmer, fishmonger, vet, air steward, etc.
Hobbies / past times
Sexual history
What is fever a sign of?
What temperature is defined as having a fever?
Fever is a sign of inflammation and can be a symptom / sign of infection
Temperature > 38.0 o C
What are the symptoms of fever?
- Fever - “burning up”
- chills, sweats, night sweats
- rigors
What is the purpose of examination in diagnosing infection?
What are examples of the 2 types of examination findings?
Examination can support diagnosis based on history
System specific findings:
- lung crackles - respiratory
- meningism - CNS
- bone tenderness - orthopaedic
- skin erythema - skin/soft tissue
- loin tenderness - urinary
Non-specific findings:
- pyrexia
- witnessed rigor / chills
- sweating
- confusion
Why might you look at haemoglobin on a full blood count in someone with a suspected infection?
Anaemia of chronic disease (normocytic, normochromic) can be caused by infection
Why might white blood cell count (WCC) be a useful indication of infection?
WCC can be raised in infection, but in other conditions too so has poor specificity
severe sepsis can lower WCC
How will the white cells vary in bacterial and viral infections?
What are the 2 inflammatory markers and how does their presence affect diagnosis?
Inflammatory markers are elements of the innate immune system
C - reactive protein and procalcitonin
Raised markers support diagnosis of infection and negative markers make infection less likely
What are normal levels of the inflammatory markers?f
C-reactive protein:
- < 5 mg/L
Procalcitonin:
- < 0.5 micro gram / L
What types of scans are used to support diagnosis of infection?
- X-rays
- computed tomography (CT)
- CT combined with PET
What tests can be used to identify severe infection?
Blood lactate and blood gases can help to identify severe sepsis and respiratory failure
What are the 3 methods of microbiological diagnosis?
- Culture
- direct detection
- immunological tests
What does the isolation of viable pathogen through culture enable?
Identification:
- immediately or by further testing
Typing:
- To identify organism relatedness
Sensitivity testing:
- to direct antimicrobial therapy
What are the 2 things that must be considered before performing culture?
- It is not applicable to non-cultivable micro-organisms
- It needs to be done before antibiotics are started
What are the 5 stages involved in blood culture sampling?
- Patient sampling
- Sample handling
- Specimen transport
- Incubation
- Growth detection
How can contamination be avoided during patient sampling?
How can sensitivity be maximised?
Contamination is minimised by aseptic technique
Sensitivity can be maximised by sampling the correct volume
What should be paid close attention to during sample handling?
Follow local instructions for safety, labelling and number of samples / bottles required
How long does the incubation phase last for?
How can microbial growth be detected?
Incubate at 35 - 37oC for 5 - 7 days
microbial growth is usually detected by constant automatic monitoring of CO2
if there is no growth, the specimen is negative and is discarded
What measurement is used in growth detection?
How long does this take?
Time to positivity (TTP) is usually 12 - 24 hours in significant bacteraemia
it may be shorter in sepsis
it may be longer in fastidious organisms
What are the final 5 stages in blood culture sampling after growth detection?
- Preliminary results
- Incubation
- Culture results
- Definitive results
- Reporting
How are the preliminary results obtained in blood culture sampling?
Why is this stage important?
A Gram film of the blood culture medium is examined and results are communicated immediately to the clinician to guide antibiotic therapy
What is involved in the 2nd incubation phase in blood culture sampling?
A small amount of the medium is incubated on a range of culture media
preliminary susceptibility testing may be carried out
What is involved in the culture results and definitive results stages of blood culture sampling?
Culture results:
- preliminary susceptibility results are communicated to the clinician
Definitive results:
- further overnight incubation is often required for definitive identification of organisms by biochemical testing, and additional sensitivity testing
What is meant by “Gram stain”?
What is the difference in colour between Gram-positive and Gram-negative?
A chemical process that distinguishes between bacterial cell walls that retain crystal violet, and those that do not, when stained and washed with acetone
Gram-positive:
- purple
Gram-negative:
- pink (or colour of counter-stain)
What is required for sensitivity testing?
Sensitivity testing requires viable micro-organisms
usually bacteria or fungi
What is the basic principle behind sensitivity testing?
- Culture of micro-organism in the presence of antimicrobial agent
- work out if the concentration of antimicrobial that will be available in the body is high enough to kill the micro-organism
- use solid or liquid media
What are the uses of sensitivity testing?
What are the 2 types of treatment that can be used as a result, and what do they require?
It informs decisions on targeted antimicrobial therapy
Initial treatment is with empiric therapy
Subsequent therapy is targeted
This requires isolation of the micro-organism and antimicrobial sensitivity testing
What is the main limitation of sensitivity testing?
The correlation between antimicrobial sensitivity and clinical response is not absolute
What are the 3 main uses of culture?
- Establishes the presence of a micro-organism at a particular site
- allows the use of empiric and targeted antimicrobial therapy
- provides epidemiological and typing information
What are the limitations of culture?
- It is only suitable for cultivable organisms
- it is usually slower than direct detection
What are the principles of direct detection?
Detection of whole organism:
- through microscopy
Detection of component of organism:
- antigen
- nucleic acid (DNA or RNA)
What is the main antigen detection test?
What are the pros and cons of this test?
Legionella antigen detection test
it can be used as a point of care test as it gives rapid results at the bedside
it requires training and quality control can be a problem
In what types of infections is a detection of nucleic acid test performed?
Viruses:
- influenza
Bacteria:
- Streptococcus pneumoniae
- through 16S PCR
Fungi:
- Candida spp.
- Aspergillus spp.
What are the 3 main uses of direct detection?
- It establishes the presence of a micro-organism at a particular site
- it can be used against cultivable and non-cultivable organisms
- it allows the use of appropriate empirical antimicrobial therapy
- usually the fastest diagnostic method
What is the drawback of direct detection?
It doesn’t give any information on antimicrobial sensitivity or typing
What is involved in an immunological test?
What are the 3 main types?
It involves the detection of immune response to infection
This is usually through antibody detection:
- IgM detection
- Seroconversion
- Fourfold rise in titre
How does seroconversion work as a method of antibody detection?
Change from negative to positive result from one test to a subsequent test
What is meant by “fourfold rise in titre” as a method of antibody detection?
The rise in concentration of antibody from one test to a subsequent test
“titre” is 1 / greatest dilution at which antibody is detected
i.e. if antibody is just detectable at a serum dilution of 1/64, then titre is 64
a fourfold rise in titre would be 2 –> 32 or 4 –> 64
Other than antibody detection, what other immunological test is used?
IFN-y release assays in tuberculosis
What are the uses of antibody testing?
It confirms exposure to a specific micro-organism
It can be used against cultivable and non-cultivable organisms
What are the limitations of antibody testing?
It is restricted to patients with a detectable antibody response
It is retrospective - it is often too late to inform antimicrobial therapy decisions
