Diagnostics

Question 1

what is sensitivity

Answer 1

(true positives)/ (true positives + false negatives)

Question 2

what is specificity

Answer 2

(true negatives)/ (true negatives + false positives)

Question 3

for ruling out syphilis:

Answer 3

• rapid plasma reagin (RPR) test
• venereal disease reference laboratory (VDRL) test
• these tests have a low false negative rate (very sensitive)

Question 4

what are the types of stain

Answer 4

gram stain and acid fast stain

Question 5

how is a gram stain done

Answer 5

• 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

Question 6

how is an acid fast stain done

Answer 6

• 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

Question 7

what bacteria are not cultured

Answer 7

rickettsias, chlamydiae, and mycoplasmas

Question 8

what are examples of open ended sampling vs looking for a particular pathogen

Answer 8

• pus specimen from brain abscess
• throat culture examined for streptococcus pyogenes

Question 9

how is a selective media prepared

Answer 9

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

Question 10

what are examples of selective media species

Answer 10

salmonella shigella agar used to isolate salmonella and shigella species

Question 11

what is differential media

Answer 11

• incorporation of certain chemicals into a medium may result in diagnostically useful growth or visible changes in the medium after incubation

Question 12

what are examples of differential media

Answer 12

eosin methylene blue agar differentiates between lactose fermenters and nonlactose fermenters
- lactose fermenters will stain dark

Question 13

what is enrichment media

Answer 13

the addition of blood, serum will support the growth of fastidious bacteria.
- used to isolate bacteria from CSF, pleural fluid, sputum and wound abscesses

Question 14

what are characteristic media

Answer 14

used to test bacteria for particular metabolic activities, products or requirements

Question 15

what is an example of characterisitic media

Answer 15

urea broth used to detect the enzyme urease, some enteric bacteria can break down urea

Question 16

what is bacteriophage typing

Answer 16

• done by CDC and certain labs
• based on specificity of phage surface molecules for host cell surface molecules
• phagovar

Question 17

what is the latex agglutination test

Answer 17

latex bead coated with antibody to particular pathogen material

Question 18

what is nucleic acid based diagnosis of infection

Answer 18

• single stranded DNA probe specific to an organism is used to identify the organism

Question 19

what can nucleic acid based diagnosis be used to probe

Answer 19

• purified DNA
• colonies
  -clinical specimens

Question 20

What is PCR test

Answer 20

-the patient sample is mixed with specific primers - dNTPs
- 3 steps: heat denaturation, annealing, synthesis

Question 21

what are microarrays

Answer 21

PCR is conducted using primers that bind to all bacterial chromosomes but amplify intervening sequences that are different in each species

Question 22

describe the test for SARS CoV-2 virus

Answer 22

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

Question 23

how is an HIV infection diagnosed

Answer 23

standard tests detect HIV antigen and anti HIV antibodies

Question 24

how is Zika virus tested for

Answer 24

• 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)