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Micro - The Appropriate Use of the Microbiology Laboratory (i) (Qualities…
Micro - The Appropriate Use of the Microbiology Laboratory (i)
Uses
Dx/confirms an infection
antimicrobial susceptibly tests - guides Tx + ID of MDR bacteria in a hospital
epidemiological info for community prevention strategies (e.g. vaccination)
Qualities of an ideal lab
rapid
accurate
comprehensive
interpretive
safe
quality-assured
daily int quality control
ext control (national audit)
international standards + guidelines
optimum time
before starting tx
in pyrexia: take blood for culture x3/day during spikes
serology now + 4 wks later (expect 4-fold increase)
correct specimen
blood for septicaemia, bacteraemia, endocarditis
clearly + appropriately labelled
sterile container for pus, fluid, abscess + tissue
swabs for body sites (wound, skin, ulcer)
specific molecular swabs for molecular tests
fresh specimen
minimises contamination
many pathogens don't survive long outside host (N gonorrhoeae, pneumococcus, haemophilus)
refrigerate if delays expected
appropriate transport medium
direct inoculation in STI clinics
viruses: contains antibiotics to prevent bacterial growth
GC agar for PCR
FLU transport medium for influenza
experienced
virology limited in Beaumont - send to reference lab
Clinician's job in pre-analytical phase
only order necessary tests
provide relevant + good quality specimens (direct better than swabs)
provided relevant clinical info (Hx) - aids + essential for interpretation
obtain specimen
processing
plate reading
interpret tests
paper reports
Lab techniques (i)
inspection
visibly cloudy/turbid CSF = highly suggestive of bacterial meningitis
microscopy
light microscopy
quick guide
CSF
urine
if epithelial cells present - contamination - reject specimen
vaginal swabs
fluid
gram stain
wounds
pus
tissue
blood
guides Tx
morphology
S aureus: gram +ve cocci in clusters ("bunch of grapes)
S pyogenes: gram +ve cocci in chains (sore throat)
Pneumococcus: gram +ve diplococci
Neisseria: gram -ve cocci
dark-field microscopy: visualises spirochetes
phase-contrast microscopy: visualises int cellular detail
fluorescent microscopy
used for serology (labelled Igs)
used for TB detection (dye = auramine O)
used for RSV detects in NP cells
UV radiation - emission of light from dye
EM
v high magnification
useful for viruses
Grocott stain
presence of fungal hyphae in lung tissue
blue
antibiotic susceptibility testing
dark diffusion
antibiotic gradient MIC
susceptible, intermediate (increase dose), resistant
internationally standardised with EUCAST
traditional methods
culture + antibiotic susceptibility testing
pros
well-established
can provide polymicrobial "catch-all" Dx
can recover living organisms
cons
labour-intensive
expensive
slow
operator dependent - subjective + indefinite