Positive (including indeterminate results)

Diagnosis

Prior

Conditional

Product

Revised

Cancer

3%

89%

267

10.7%

No cancer

97%

23%

     2,231

89.3%

 

 

 

Σ = 2,498

 

The revised probability after a positive or indeterminate result

would be only 10.7 percent, but 24.98 percent of patients would proceed to surgery (compared to 5.46 percent, as calculated above for positive FNA results) whereas 75.02 percent would avoid surgery.

In contrast, if an indeterminate result were treated as negative (i.e., the work-up stops), then the revised probability of cancer would be 0.4 percent because of false negatives. In this case, 82.78 percent of patients would avoid surgery (compared to 75.02 percent above).

Negative (including indeterminate results)

Diagnosis

Prior

Conditional

Product

Revised

Cancer

3%

11%

33

0.4%

No cancer

97%

85%

      8,245

99.6%

 

 

 

Σ = 8,278

 

In a similar vein, consider the effect of an unsatisfactory FNA. Because cancers are more likely to provide satisfactory samples, an unsatisfactory result lowers the probability of cancer, as shown above, from 3 percent to 1 percent. Recall that a negative FNA only lowers the probability of cancer to 0.3 percent (because of the possibility of false-negative results).



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