Diagnosis Flashcards
Name 4 reasons to diagnose
- Identify aetiological agent
- Immunity to infection
- Prognosis
- Epidemiology
What are 3 components of prognosis?
- Monitor response to treatment
- Monitor and prevent infection
- Detect antiviral resistance
Describe the diagnostic chain
- Clinical indication
- Specimen collection
- Specific request
- Transport to Lab
- Logged-in
- Initial processing
- Test procedure
- Data retention
- Result authorisation
- Report generation and transfer
- Clinical action
What 2 things can be involved in processing a sample?
- Inactivating virus present
2. Spinning down blood
Why is specimen collection one of the most important step in diagnosis?
Results of diagnostic tests for infection diseases are dependent on selection, timing and method of collection
Name 4 important test characteristics
- Sensitivity
- Specificity
- Positive predictive value
- Negative predictive value
What is sensitivity of a test?
Proportion of people with the disease who test positive
What is specificity of a test?
Proportion of people without the disease who have a negative test
What is positive predictive value of a test?
Proportion of people testing positive who actually have the disease
What is negative predictive value of a test?
Proportion of people testing negative who do not have the disease
What are 6 features of a good diagnostic test?
- Accurate
- Reliable
- Reproducible
- Timely
- Affordable
- Available
What are 5 methods of diagnostic microbiology?
- Microscopy
- Culture
- Immunological
- Molecular methods
- Point of care tests
How is immunological microbiology usually carries out?
Sample of blood
What are 3 types of microscopy?
- Light
- Fluorescent
- Eletron
Describe 3 things to look for during light microscopy
- Staining characteristics
- Shape
- Size
What is fluorescent microscopy?
Specimens stained with fluorescent dye which emits light when excited under fluorescent microscope e.g. auramine staining of TB bacilli
Describe electron microscopy
- Uses electron beam to produce magnified image
- Achieves better resolution and magnifications
- Limited sensitivity, expensive and specialist
- Diagnosis of smallpox (bioterrorism)
What are 3 characteristics of bacteriological culture growth?
- Clinical specimen inoculated onto various media
- Appearance of colonies suggest species presence
- Further tests to clarify
What 3 ways can provide various media for inoculation?
- Selective agar
- Nutrient enrichment
- Incubation at different temperatures and atmospheres
What type of bacteriological culture can help identify streptococci?
Haemolysis as pathogen lysis red blood cells
Describe how antimicrobial sensitivities can be tested on an agar plate
- Filter paper disc is impregnated with antibiotic
- Incubate bacteria on agar with discs placed on top
- Area of zone of inhibition around the disc shows sensitivity
What does a VITEK identification card consist of?
30 wells each containing biochemical substrates in a dehydrated form
What does a VITEK susceptibility testing card consist of?
30 or 45 wells containing dehydrated antibiotics with specific tests for detecting resistance mechanisms are systematically included
What does MALDI-TOF stand for?
Matrix-assisted laser desorption/ionization time-of-flight
What is MALDI-TOF Mass Spectrometry?
Ionization method which uses a laser energy absorbing matrix to create ions from large molecules with minimal fragmentation
Describe the 3 step process of MALDI-TOF Mass Spectrometry
- Sample is mixed with suitable matrix material and applied to metal plate
- Pulsed laser irradiates sample, triggering ablation and desorption of sample and matrix
- Analyte molecules ionized in the hot plume of ablated gases, and can be accelerated into whichever mass spectrometer is used to analyse them
Describe the bacteriological diagnosis of streptococcal pharyngitis
- Throat swab
- Complete haemolysis on blood agar
- Latex agglutination typing test
Describe the bacteriological diagnosis of candida albicans
- Culture in germ tubes
Describe 2 forms of bacteriological diagnosis of dermatophyte infections
- Direct microscopic examination of scales dissolved in KOH
- Culture of scrapings of lesions
What are 5 characteristics of serology?
- Testing for specific microbial antigens
- Direct detection from clinical specimens
- Characterization of cultured organism
- Testing for antibody to specific microbial antigens
- Detection of particular isotope (IgM or IgG)
What different types of infection are suggested by presence of IgM or IgG isotopes in serolgy?
- IgM suggests acute infection (IgG may or may not be present)
- IgG suggests long term or past infection (IgM will not be present)
How is serological technique carried out?
- Add specific viral antigen to patient serum
- If virus specific antibody is present an antigen-antibody complex will form
- Indicator system is used to detect if complex has formed
What is an enzyme immunoassays?
Anti-human antibody combined with enzymes which will react with antigen to produce a colour change
What is the incubation period of a virus?
Time to onset of symptoms
What is the window period of a virus?
Time between start of symptoms and development of an antibody
Why is a blister swabbed in chicken pox and not blood?
- It takes 14 days for an onset of symptoms so if blister swabbed at 14 days virus is present
- It takes 19 days for an antibody to be produced so if blood sample taken before 19 days negative test will occur
Name 2 main types of molecular techniques used to detect viral genomes
- Real-time PCR
2. Sequencing and next generation sequencing
What are 3 characteristics of real time PCR?
- Allow detection and quantification of DNA sequence based on use of fluorescent reporter
- Fluorescence is measured during each cycle and is proportional to amount of PCR produce
- No genome present means fluorescence does not occur / Lots of virus present means lots of fluorescence
Describe the steps of real time PCR
- Specific probe to sequence within PCR product is labelled with reporter dye on one end and quencher dye on the other
- Taq polymerase come across bound probe
- 5’ exonuclease degrades probe releasing reporter dye to fluoresce
What is DNA sequencing?
Process of determining nucleic acid sequencing, including any method or technology used to determine the order of the four nucleotides in DNA
What is NGS?
Next generation sequencing which is the catch-all term used to describe a number of different modern sequencing technologies
What is point of care testing?
Diagnostic testing at or near the point of care i.e. at the time and place of patient care (not labratory)
Give 4 examples of POCTs
- HIV testing in GUM clinics
- Helicobacter pylori testing in endoscope clinics
- Influenza testing in A&E
- Strep throat
What is the main advantage of POCTs?
Faster results for better patient management
