Week 3- Labs Flashcards
Why do we order tests?
a) Used to diagnose in patients known to be sick
b) Provide prognostic information on patients with known disease
c) Identify subclinical disease or risk of developing disease
d) Monitor ongoing therapy
Decision making process for appropriate lab tests.
a) History and physical examination leads to differential diagnoses
b) Can be ranked by probability
i) Estimate pretest probability based on patient characteristics
ii) Revise probability based upon new information (diagnostic results)
iii) Know where the probability thresholds are (what are the risks associated with the decision to treat)
Define Incidence and Prevalence.
a) Incidence: The number of new cases of a disease that occur during a specified period of time in a population at risk for developing the disease (a measure of risk)
b) Prevalence: The number of affected persons present in the population at a specific time divided by the number of persons in the population at that time.
c) Point prevalence and period prevalence
d) Incidence and prevalence are useful in determining accurate diagnoses and treatments
Define “Gold Standard”
a) The best test or combination of tests which are considered to be the preferred method of diagnosis
b) Any new test must be compared to the gold standard
c) The gold standard is different for different disease states
Define Validity. How is it measured?
a) Extent to which a test measures what it is supposed to measure
b) The accuracy of the test to determine if a disease is present or not
c) Measured by sensitivity and specificity
Define Positives: True and False.
a) True positive: patient has the disease and the test is positive
b) False positive: Patient does not have the disease but the test is positive
Define Negatives: True and False.
c) True negative: Patient does not have the disease and the test is negative
d) False negative: Patient has the disease but the test is negative
Define Sensitivity.
a) Equals the number of true positives for a test divided by the number of tested individuals who truly have the disease
b) The ability of a test to correctly classify a person as having a condition
c) Positive in disease
True positives_______
True positives + False negatives
Define Specificity.
a) Greater when a test has fewer false negatives
b) Equals the number of true negatives divided by the number of all tested individuals who do not have the disease
c) Negative in health
d) Inversely proportional to sensitivity
e) The ability of the test to correctly identify patients without a disease
True negatives______
True negatives + False positives
Define Predictive Value: Positive and Negative.
a) Positive predictive value (PPV) equals true positives divided by all positives
b) Negative predictive value (NPV) equals true negatives divided by all negatives
c) Predictive values are dependent upon prevalence of the condition
d) As disease prevalence increases the positive predictive value will increase
Define Liklihood Ratio.
a) How much more likely it is that a patient who tests positive for the disease has the disease compared to one who tests negative
b) Likelihood ratio = ____Sensitivity_______
1- Specificity
Examples of Dependence of PPV and NPV on Disease Prevalence.
a) Screening for Systemic Lupus Erythematosis (SLE) has a low PPV due a high number of false positives BUT if patient is symptomatic PPV raises because the population of individuals with symptoms the prevalence is higher
b) D-dimer in post-partum woman with shortness of breath has low PPV for diagnosing PE because D-dimer is typically elevated in this population BUT D-Dimer has high NPV since a low D-dimer is likely NOT associated with PE
In a perfect world, describe the accuracy of test results.
a) All tests would be 100% accurate 100% of the time
b) Good enough: Providing treatment to patients who test positive with a highly sensitive/low specificity test
c) Along with that, the patient should test negative to a low sensitive/highly specific test
Additional information/considerations for lab testing.
a) Testing is based upon presentation, assessment, and possible differential diagnoses
b) Testing is used to direct pharmacotherapy
c) As we move forward, we will at times be discussing specific labs within class and discussions
Summary of reason for labs
a) Labs and other diagnostics can be valuable tools to establish the presence or absence of a condition
b) Diagnostics should be used to augment care provided, and assist with the treatment plan, not replace patient contact and practitioner assessment
c) Knowledge of why we would provide tests will help to determine what and when we would test
