The results of non-invasive tests (e.g. ultrasound) can be highly variable, often providing ambiguous results. Although other parameters can be reviewed, calculation of overall accuracy, sensitivity and specificity as well as positive and negative predictive values are useful to the clinician who is managing the patient.
To calculate these statistics, ultrasound results must be compared to the established gold standards, usually angiography, surgery or autopsy findings. The simplest statistic compares the outcome of each test as either positive or negative. A true-positive result indicates that both tests are positive. A true-negative result indicates that both tests are negative. A false-positive result means that the gold standard is negative, indicating the absence of disease, while the non-invasive study is positive, indicating the presence of disease. A false-negative result occurs when the non-invasive test indicates the absence of disease but the gold standard is positive. True-positive and true-negative results can be used to calculate sensitivity and specificity. Sensitivity is the ability of a test to correctly diagnose disease. It can be calculated by dividing the number of true-positive tests by the total number of positive results obtained by the gold standard.
Specificity is the ability to diagnose the absence of disease and is calculated by dividing the true negative by the total number of negative results obtained by the gold standard. The positive predictive value (PPV) or likelihood means that disease is present and negative predictive values (NPV) means that disease is not present.