diagnosis of infection Flashcards
specimen types
urine swab blood tissue sputum feaces
1)direct detection
detection of whole organism by microscopy
detection of component e.g antigen, nucleic acid
microscopic
CSF in meningitis
gram negative diploccoci = neisseria
antigen
clostridium difficile glutamata dehydrogenase antigen toxin detected using lateral flow
nucleic acid
Neisseria meningitidis uses direct sequence specific target
streptoccocus pneumoniae amplify unknown sequence using 16s rRNA match with sequence in database
uses of direct detection
establishes present of organism at one site tests for non cultivable organisms directs empirical therapy fast epidemiological info
limitations of direct detection
no info on susceptibility testing
2) culture
isolation of the pathogen allows
identification
typing
susceptibility testing
different culture media
clostridium perfringens requires anaerobic
mycobacterium obligate aerobe
bordetella pertussis is fastideous - fresh blood and potato starch
fresh blood agar
non selective
nutricious
mannitol salt agar
selects for halophile growth and pH indicator for fermenters
MacConkey agar
bile salts select for enteric organisms
pH indicator detects lactose fermenter - turns red - E. coli
VPAT - selective for neisseria gonorrhea and campylobacter jejuni
gram stain
gram positive = purple = peptidoglycan
negative = counter stain
haemolysins
streptococcis
a haemolysins - strep pneumonia (resistant to optochin), strep viridans (oral)
B haemolysis - strep pyogenes
oxidase test
test for cytochromes
pseudemonas positive = purple
fermenter = no cytochromes = negative = clear = e coli
catalase test
differentiates streptococci with staphylococci
staph = positive = bubbles
coagulase
staph aureus is positive
biochemical tests
AI strips mini biochemical reactions
octal score looked up in a database
maldi tof
time of flight analysis
mass spectrometer produces spectrum of bacterial proteins
compared to database of known spectra to produce ID
culture uses
targeted antimicrobial therapy
limitations of culture
cultivable organisms only
succeptibility testing
viable organism
measure zone of inhibition
E- test gives MIC
BSAC
3) immunological tests
detection of an immune response to infection
antibody detection
ELIZA
IgM permits timing from initial infection
seroconversion
requires a fourfold rise in antibody titre ie present at both 1/2 and 1/64
IFN-y release assay
Tuberculosis
antibody detection uses and limitations
confirms exposure to antigen
epidemiological info
restricted to patients with detectable antigen response - not in immunocomprimised
retrospective - usually too late to treat if immune response high
