 How do I know whether a test is really useful—whether it will really shift the probability of disease across a threshold?

A perfect diagnostic test would always be positive in patients with the disease and would always be negative in patients without the disease (Figure 1-6). Since there are no perfect diagnostic tests, some patients with the disease have negative tests (false-negative), and some without the disease have positive tests (false-positive) (Figure 1-7).

###### Figure 1-6.

A perfect diagnostic test. FN, false negative; FP, false positive; TN, true negative; TP, true positive. ###### Figure 1-7.

A pictorial representation of test characteristics. FN, false negative; FP, false positive; TN, true negative; TP, true positive. The test characteristics help you to know how often false results occur. They are determined by performing the test in patients known to have or not have the disease and recording the distribution of results (Table 1-3).

Table 1-3.Test characteristics.

Table 1-4 shows the test characteristics of duplex ultrasonography for the diagnosis of proximal DVT, based on a hypothetical group of 200 patients, 90 of whom have DVT.

Table 1-4.Results for calculating the test characteristics of duplex ultrasonography.

The sensitivity is the percentage of patients with DVT who have a true-positive (TP) test result:

Sensitivity = TP/total number of patients with DVT = 86/90 = 0.96 = 96%

Since tests with very high sensitivity have a very low percentage of false-negative results (in Table 1-4, 4/90 = 0.04 = 4%), a negative result is likely a true negative.

The specificity is the percentage of patients without DVT who have a true-negative (TN) test result:

Specificity = TN/total number of patients without DVT = 108/110 = 0.98 = 98%

Since tests with very high specificity have a low percentage of false-positive results (in Table 1-4, 2/110 = 0.02 = 2%), a positive result is likely a true positive.

The sensitivity and specificity are important attributes of a test, but they do ...

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