infection tests Flashcards
what are infection tests for?
optimal treatment and prevention of infection
why would you carry out a diagnostic test?
to improve outcome or to provide epidemiological data
how is a diagnostic hypothesis reached?
through history, examination, non - microbiological tests, lab tests
what is direct detection?
it is detection of a whole organism, antigen or genomic material
what does serology detect?
an antibody response
what is fever a sign of?
a sign of inflammation and can be a sign of infection
what is the most common cause of community acquired pneumonia?
streptococcus pneumoniae
`how is Hb related to infection?
can be anaemia of chronic disease - normocytic or chromic caused by infection
what is the use of WCC?
can be raised in infection and other condition and therefore is not specific. Severe sepsis can lower WCC
what are the levels of markers in bacterial infection?
WCC raised, lymphocytes are normal or low, neutrophils are raised
what are the levels of markers in viral infection?
normal WCC, raised lymphocytes and normal neutrophils
what are the inflammatory markers of the innate immune system?
procalcitonin (0.5micrograms/L) and CRP (<0.5mg/L) - raised means diagnosis of infection
what imaging is used to support a diagnosis of respiratory infection?
chest Xray, CT and CT combined with PET
what two tests can help to identify severe sepsis and respiratory failure?
blood gases and blood lactate
what comprises biological diganosis?
culture, direct detection and immunological tests
what needs to be done for ABs are started?
culture
what can culture do?
it enables the identification of organisms by immediate or further testing, typing to establish organism relatedness and sensitivity testing to direct antimicrobial therapy
what is culture?
it is the isolation of viable pathogens
what is the process of sampling?
patient sampling, sampling handling, sampling transport incubation, growth detection , preliminary results, incubation, culture results, definitive results and then reporting
what is considered in patient sampling?
the aseptic technique to reduce contamination and the optimal volume for sensitivity
what will local sample handling guidelines tell you?
the safety, labelling, number of samples and bottles required
when should the samples be transported to the lab?
as soon as possible
what happens in the incubation stage?
incubate at 35-37 degrees for 5-7 days and detect microbial growth through the constant automatic monitoring of CO2 - if no growth then the specimen is negative and discarded
what is used in growth detection?
TTP - time to positivity
TTP is usually 12-24 hours for significant bacteriaemia
shorter if overwhelming sepsis
longer if more fastidious organism
what occurs during and after preliminary resultd?
in preliminary a gram stain is observed and then incubation means that the pathogen is incubated on a range of culture media for susceptibility testing to guide antimicrobial therapy
what occurs in culture results?
the results of the tests are communicated to the clinician
what happens in definitive results?
overnight incubation
identification using biochemical testing, additional susceptibility testing, identification by MALDI-TOF
what is reported?
final summary for clinical care and communication
what is gram stain?
it is a chemical process that distinguishes between the bacterial cell walls that retain crystal violet and those that do not when stained and washed with acetone
what will a positive gram stain show?
purple
what will a gram negative stain show?
pink or the colour of the counter stain
what does sensitivity testing require?
viable organisms - usually bacteria or fungi
liquid or solid media
what is the basic principle of sensitivity testing?
it is the culture of a microorganism in the presence of an antimicrobial agent to work out if the concentration of the antimicrobial will be high enough in the body to kill the micro-organism
what is the role of sensitivity testing?
to inform decisions on targeted antimicrobial therapy so that you can start with empiric and then work towards targeted therapy in subsequent treatments
what does sensitivity testing require?
isolation of the microorganism and antimicrobial susceptibility testing
what are the uses of sensitivity testing?
establishes the presence of a microorganism at a particular site, provides epidemiological data and typing information and allows the use of empiric and targeted therapy
what are the limitations if sensitivity testing?
cultivable organisms only, is usually slower than direct testing and the correlation between antimicrobial sensitivity and clinical response is not absolute
what is direct detection?
it is detection of the whole organism using microscopy or detection of a component of the organism using an antigen or nucleic acid
what is an example of an antigen detection?
legionella antigen detection test
what are the issues with antigen detection tests?
although they can be done at bedside and give quick results they require training to do and quality control can be an issue
what types of antigens and give an example can be targeted?
polysaccharide capsules such as in neoformans, cell wall polysaccharides such as in aspergillus
what is a limitation of the antigen test itself?
the solubility and distribution of an antigen will differ with different infecting species and this is poorly understood for aspergillus and candida
what are used in serological tests?
latex agglutination in neoformans and ELISA in candida and aspergillus
what is considered in the sampling protocol?
whether it should be screening or diagnostic, whether the diagnosis influences clinical outcome and whether the result offer any advantage over other methods
what viruses, bacteria and fungi can be detected through nucleic acid detection?
virus - influenza
bacteria - strep pneumoniae
fungi - candida and aspergillus using 16S PCR
what are the benefits of direct detection?
it establishes the presence of an organism at a particular site, can be cultivable and non cultivable organisms and allows the use of the appropriate empiric theory
what are limitations of direct detection?
it does not give any information on antimicrobial susceptibility and testing is usually the fastest diagnostic method
what are immunological tests?
the detection of the immune response to infection - IgM or ABs, or using seroconversion
what is seroconversion?
seroconversion is the time period during which a specific antibody develops and becomes detectable in the blood
what will happen over seroconversion?
there will be a change from a negative to a positive result in subsequent tests, a four fold rise in titre - a rise in the concentration of AB from one test to following
what is titre?
it is the greatest dilution at which antibody is detectable
what is an example of an immunological test?
IFN-y release assays in TB
what are the uses of antibody testing?
confirms the exposure to a specific microorganism and can be cultivable and non cultivable organisms
what are the limitations of antibody testing?
it is restricted to patients with a detectable antibody response and is retrospective and therefore may be too late to inform the antimicrobial therapy decisions
