Diagnostic & Screening Tests in Respiratory Disease Flashcards
What is the difference between a diagnostic test and a screening test?
Diagnostic Test:
Purpose: confirmation of disease or otherwise ie, establish diagnosis
- results are (mostly) definitive
- applied to patients in whom there is clinical suspicion of disease - ie, high pre-test probability of disease
Screening Test:
Purpose: identification of patients who may have disease
- results are preliminary - need confirmation with diagnostic (definitive) test(s)
- applied to people in whom there is no clinical suspicion of disease - ie, lower pre-test probability of disease
What are sensitivity and specificity?
sensitivity = TP / TP+FN = % people with disease that test positive
specificity = TN / TN+FP = % people without disease that test negative
What is the difference between a positive predictive value and a negative predictive value?
positive predictive value = TP / TP+FP = % positive tests that is truly positive
negative predictive value = TN / TN+FN = % negative tests that is truly negative
Explain the features surrounding underlying prevalence of disease.
- sensitivity and specificity are inherent to a test and are constant
- PPV and NPV are dependent on:
-sensitivity and specificity
-underlying prevalence of disease - PPV positively correlated with underlying prevalence of disease (and NPV negatively correlated with underlying prevalence)
- utility of diagnostic/screening test
dependent on prevalence of disease - order diagnostic tests only if there is strong clinical suspicion of disease
- do not use diagnostic tests to screen for diseases
- screening tests need to be targeted to ‘high risk’ individuals - eg, mammography in women >50yr
What is meant by liklihood ratios?
- likelihood that a given test result would be expected in a patient with the disease compared to a patient without the disease
- LR of positive test = sensitivity / (1-specificity)
- LR of negative test = (1-sensitivity) / specificity
Explain the relationship between specificity and sensitivity?
The trade-off between sensitivity and specificity is that most test results expressed on continuous scale, and arbitrary thresholds define presence of disease - eg, FEV1 and COPD however disease states are often not absolute
- lower threshold: ↑ sensitivity, ↓ specificity - higher threshold: ↓ sensitivity, ↑ specificity
What is the purpose of a Receiver Operator Characteristic (ROC) Curve?
- plot: 1-specificity vs sensitivity for various thresholds (cut-off values) for a test
- graphical representation of trade-off between sensitivity and specificity in tests
- indicates how well a test discriminates people with disease from people without disease
- ideal test (full discrimination): 100% sensitivity and 0% 1-specificity (100% specificity) - ie, test picks up everyone with disease and no-one without
- no discrimination: perfect correlation between sensitivity and 1-specificity - ie, test just as likely to ‘pick up’ people with disease as people without
What is meant by screening?
- key preventive strategy
- rationale: early detection better outcomes
- assessment of a population to identify:
- risk factors (primary prevention)
- early disease (secondary prevention)
- undertaken on largely healthy people
What are The WHO Criteria for Screening?
The WHO Criteria for Screening: • important health problem • natural history well understood • detectable early stage • early treatment beneficial • suitable test for early disease • acceptable test • intervals for testing determined • adequate healthcare provision for extra workload • risks (including psychological) less than benefits • costs balanced against benefits
What are some limitations of screening?
There are some limitations for Screening:
- inaccuracy of screening tests
- may not be cost-effective
- physical and psychological side-effects
- biases in measurement of effectiveness
- selection - healthy more likely to be screened
- lead-time - early detection, not prolonged survival
- length-time - detection of non-aggressive disease
