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PART 6 — MISCELLANEOUS
measurements employed to compare testing results to a gold standard.
True Positives
True positives (TPs) are the number of studies per- formed by ultrasound, which state that disease is present, and the gold standard agrees with the ultra- sound findings. For example, an ultrasound shows a dilated noncompressible femoral vein with no flow. The venogram demonstrates an occluded femoral vein, and the physician’s diagnosis is a deep venous thrombosis (DVT). All are in agreement that disease is present; therefore, a true positive.
True Negatives
True negatives (TNs) are the number of negative find- ings reported by ultrasound that were also reported negative by the gold standard. For example, the ul- trasound indicates no evidence of echogenic plaque and normal flow velocities in the proximal internal carotid artery. An arteriogram does not identify any disease, and the physician’s interpretation is no inter- nal carotid artery disease. Thus, all are in agreement that no disease is present; therefore, a true negative.
False Positives
Unfortunately, there are some cases where the ultra- sound and gold standard do not agree. When using arteriography as the gold standard, in almost every case, those findings represent the correct findings. There may be a situation where the duplex ultrasound demonstrates velocity elevations consistent with a stenosis, but the arteriogram finds no disease. Another example may be a case where the sonographer fails to properly compress a vein, which is then interpreted as an occluded vein. Subsequent venography dem- onstrates a patent, thrombus-free vessel. Fortunately, disagreements are relatively rare but statistics is nec- essary to help identify these inaccuracies. When these mismatches occur, studies can be carefully reviewed to determine the specific source of the error.
The examples in the preceding paragraph are false positive (FP) results. False positives are studies that are reported positive but found to be negative by the gold standard. Another variation of the FP result could occur when a carotid ultrasound study met the criteria for a 50% to 79% stenosis, but the arteriogram only detected a 40% stenosis. This is a false positive; however, it is important here for both radiology and ultrasound to be clear about what an FP constitutes. A difference of 5% error in velocity measurement could put the findings in one category or another. Although the categories are significantly different and could affect medical manage- ment, the actual difference is relatively small.
False Negatives
False negatives (FNs) state that a study was normal when the gold standard identifies disease. An exam- ple of a false negative would be if a normal aortic di- ameter is reported by ultrasound, but the arteriogram shows a tortuous aorta with a 3.0 cm abdominal aortic aneurysm (AAA). Some might argue that this may be a relatively minor error and would have little clinical significance. But a report stating “No DVT in femoral vein” that turns out to be a false negative may be life threatening. Clearly, the sonographer wants to have a high degree of true positives and true negatives, with no or very few false negatives and false positives.
ACCURACY
QA statistics use the calculations of TN, TP, FN, and FP in order to determine other indicators that describe the results of a test. Accuracy is one such indicator. Accuracy can be thought of as the degree of “close- ness” of something to its actual value. For example, in the vascular laboratory, we gather imaging and Doppler data to identify carotid artery disease into a category such as 50% to 79%, or 80% to 99% ste- nosis. We typically then compare our results (duplex findings) with the gold standard (arteriogram results) to see how “close” the ultrasound findings are to the actual value (degree stenosis). Accuracy is the per- centage of correct results. Not only is it important to be accurate in identifying disease when it is present, but also to identify normal vessels when disease is absent. Accuracy is calculated as the total number of correct tests divided by the total number of all tests.
SENSITIVITY
Sensitivity measures the proportion of actual positives studies, which are correctly identified. It is the abil- ity to identify disease when disease is present. When positive ultrasound studies correlate closely with pos- itive arteriogram findings, the test is said to have good sensitivity. Ultrasound is very good at determining if DVT is present in the lower extremities but is of no use in detecting pneumonia in the lungs. Here, one might say that ultrasound is very sensitive for DVT, but has no sensitivity for detecting pneumonia.
Sensitivity is calculated by taking the true posi- tives results and dividing these by the all-positive results as determined by the gold standard.
SPECIFICITY
As sonographers, we know it is not only important to identify disease when it is present but also to be certain that when we fail to find disease, no disease truly exists. Physicians rely heavily on the ultrasound reports and, more often than not, manage the patient