Diagnostics Flashcards
what is sensitivity
(true positives)/ (true positives + false negatives)
what is specificity
(true negatives)/ (true negatives + false positives)
for ruling out syphilis:
- rapid plasma reagin (RPR) test
- venereal disease reference laboratory (VDRL) test
- these tests have a low false negative rate (very sensitive)
what are the types of stain
gram stain and acid fast stain
how is a gram stain done
- apply the primary stain- crystal violet_ to a heat fixed smear of bacteria
- add a trapping agent- Grams iodine
- decolorize with acetone or alcohol. thick petidoglycan layer of gram positives keeps crystal violet iodine complex trapped within cells
- counterstain with safranin to stain decolorized cells
how is an acid fast stain done
- apply the primary stain- fuschin- and mordant to a heat fixed smear of bacteria. place piece of absorbant paper soaked with carbolfuschin over the smear and heat the slide to drive the stain+ mordant into cells
- decolorize with dilute acid in alcohol. the carbolfuschsin will wash out of most cells but not those with high levels of mycolic acid in their membranes
- counterstain with methylene blue to stain decolorized cells
what bacteria are not cultured
rickettsias, chlamydiae, and mycoplasmas
what are examples of open ended sampling vs looking for a particular pathogen
- pus specimen from brain abscess
- throat culture examined for streptococcus pyogenes
how is a selective media prepared
by the addition of specific substances to a culture medium that will permit the growth of one group of bacteria while inhibiting growth of some other groups
what are examples of selective media species
salmonella shigella agar used to isolate salmonella and shigella species
what is differential media
- incorporation of certain chemicals into a medium may result in diagnostically useful growth or visible changes in the medium after incubation
what are examples of differential media
eosin methylene blue agar differentiates between lactose fermenters and nonlactose fermenters
- lactose fermenters will stain dark
what is enrichment media
the addition of blood, serum will support the growth of fastidious bacteria.
- used to isolate bacteria from CSF, pleural fluid, sputum and wound abscesses
what are characteristic media
used to test bacteria for particular metabolic activities, products or requirements
what is an example of characterisitic media
urea broth used to detect the enzyme urease, some enteric bacteria can break down urea
what is bacteriophage typing
- done by CDC and certain labs
- based on specificity of phage surface molecules for host cell surface molecules
- phagovar
what is the latex agglutination test
latex bead coated with antibody to particular pathogen material
what is nucleic acid based diagnosis of infection
- single stranded DNA probe specific to an organism is used to identify the organism
what can nucleic acid based diagnosis be used to probe
- purified DNA
- colonies
-clinical specimens
What is PCR test
-the patient sample is mixed with specific primers - dNTPs
- 3 steps: heat denaturation, annealing, synthesis
what are microarrays
PCR is conducted using primers that bind to all bacterial chromosomes but amplify intervening sequences that are different in each species
describe the test for SARS CoV-2 virus
real time reverse transcription polymerase chain reaction or sequencing of reverse transcribed SARS CoV2 RNA genome regions
- use nasal swab or saliva
- sensitive and specific
how is an HIV infection diagnosed
standard tests detect HIV antigen and anti HIV antibodies
how is Zika virus tested for
- real time reverse transcription polymerase chain reaction
- use serum or urine collected within 2 weeks after symptoms onset
- a positive rRT-PCR result confirms Zika virus infection and no additional testing is indicated (-sensitive and specific but limited time window)
- a negative rRT-PCR does not exclude Zika virus infection and serum should be analyzed by IgM antibody testing (- sensitive with longer time window but not as specific)
