Infectious Disease Diagnostics Flashcards
what are the indications for using a diagnostic test
- determine presence/absence of disease
- gauge the severity of disease
- monitor response to therapy
- inform prognosis
sensitivity
percentage of true positives out of all disease positives
rules OUT disease - can trust the negative result
high false positive rate
Se = TP / TP + FN
specificity
percentage of true negatives out of all disease negatives
rules IN disease - can trust the positive result
high false negative rate
Sp = TN / TN + FP
positive predictive value
proportion of the true positives out of all test positives
PPV = TP / TP + FP
negative predictive values
proportion of the true negatives out of all test negatives
NPV = TN / TN + FN
PPV and NPV of diseases with high prevalence
high PPV
low NPV
PPV and NPV of diseases with low prevalence
low PPV
high NPV
categories of diagnostics tests
- organism detection
- antibody detection
what do organism detection tests do
assess for direct evidence that an infectious organism is present in your patient
whole organism, antigen, nucleic acid
what are the 4 organism detection tests
- cytology/histopathology
- culture
- antigen assays
- nucleic acid assays
what does cytology and histopathology look for
whole organsism
what does culture look for
whole organisms
can do antimicrobial susceptibility testing
what do antigen assays look for
organism proteins
what do nucleic acid assays look for
organism DNA or RNA
indications for PCR testing
- difficult or dangerous to culture organisms
- require rapid results
- acute disease
- immunocompromised aniamls
qualitative vs quantitative PCR
qualitative: determines whether or not DNA/RNA is present
quantitative: determines how much DNA/RNA is present
advantages of organism detection tests
- implies presence of organism
- can localize the disease process
- sensitive in immunocompromised groups
- possible to quantify organism number
disadvantages of organism detection tests
- false positives possible
- positive doesn’t always imply the organism is the cause of the disease
- no sense of chronology
- low sensitivity for some infections
what do antibody detection tests do
assesses the body’s response to infection
antibodies MUST be present to be detected
single vs paired titers
single: good for chronic/persistent disease
paired: good for acute disease
disadvantages of antibody detection tests
- negative for early acute diseases or localized disease
- negative in immunocompromised patients
- false positives are common
- poor indicators of treatment success
indications for organism tests
acute disease
immunocompromised hosts
indications for antibody tests
chronic/persistent disease
organisms undetectable
uses for highly sensitive tests
rule out disease (screening)
uses for highly specific tests
rule in disease (confirm diagnosis)
