Validity Testing

Gold Standard

Well-established, reliable diagnostic method used as a reference

Gold Standard for Vascular Imaging

Angiography

Validity

Ability of a test to distinguish between who has the disease and who does not

True Positive (TP)

Those who have disease and a positive test

Ultrasound shows disease, gold standard shows disease

True Negative (TN)

Those who do not have disease and a negative test

Ultrasound shows no disease, gold standard shows no disease

False Positive (FP)

Those who do not have disease and a positive test

Ultrasound shows disease, gold standard shows no disease

False Negative (FN)

Those who have disease and a negative test

Ultrasound shows no disease, gold standard shows disease

Sensitivity

Ability of test to correctly detect patients with disease compared to the gold standard - positive sonogram with positive gold standard

Sensitivity Formula

Number of true positive tests

Number of all positive tests by gold standard

Specificity

Ability of test to correctly detect patients without disease compared to the gold standard - negative sonogram with negative gold standard

Specificity Formula

Number of true negative tests

Number of all negative tests by gold standard

Positive Predictive Value (PPV)

Probability that a positive test result reflects the actual presence of disease

Portion of patients with a positive test that have disease

Positive Predictive Value (PPV) Formula

Number of true positive tests

Number of all positive noninvasive tests

Negative Predictive Value (NPV)

Probability that a negative test result reflects the actual absence of disease

Portion of patients with a negative test that do not have disease

Negative Predictive Value (NPV) Formula

Number of true negative tests

Number of all negative noninvasive tests

Accuracy

Degree of closeness of a test result to the actual value

Percentage of overall correct results

Must lie between sensitivity & specificity and PPV & NPV

Accuracy Formula

Total number of correct tests

Total number of all tests

Reliability

Consistency of obtaining similar results under similar conditions - reflects accuracy over time

Increasing Cut-Off Values

Improves specificity

More true negatives

Decreasing Cut-Off Values

Improves sensitivity

More true positives

Why is Validity Testing Important?

Required part of accreditation for labs

Ensures reason, quality, & completeness of exams

Ensures lab follows protocol & quality standards