A 45-year-old woman comes to the office with a palpable thyroid node. Medical history reveals that she was diagnosed with medulloblastoma during childhood and treated with chemotherapy and external beam radiation. Fine-needle aspiration of the thyroid shows no malignant cells. As the test results are explained to the patient, she asks, "What are the chances that I really do not have thyroid cancer?" Which of the following diagnostic test parameters would be most useful for answering this patient's question?

Question

A 45-year-old woman comes to the office with a palpable thyroid node.  Medical history reveals that she was diagnosed with medulloblastoma during childhood and treated with chemotherapy and external beam radiation.  Fine-needle aspiration of the thyroid shows no malignant cells.  As the test results are explained to the patient, she asks, "What are the chances that I really do not have thyroid cancer?"  Which of the following diagnostic test parameters would be most useful for answering this patient's question?

Quizplus · Accepted Answer

Negative predictive value (NPV) is defined as the probability of not having a disease when the test result is negative. NPV is calculated as the proportion of true negatives divided by the total number of negative tests (true and false negatives); therefore, it varies with the prevalence of disease in the target population. The prevalence of a disease in a population may be used as an estimate for the pretest probability of having the disease in patients who closely resemble the population. A patient with a high pretest probability will have a low NPV with a negative test, whereas a patient with a low pretest probability will have a high NPV with a negative test. This patient has a high pretest probability for having thyroid cancer since she most likely received significant radiation exposure to the thyroid during childhood. As such, her negative cytology results would have a lower NPV when compared to another patient with no risk factors for thyroid malignancy who also tests negative. (Choice B) Positive predictive value also varies according to the pretest probability but applies to positive test results. (Choices C and D) Sensitivity and specificity of a test are intrinsic characteristics of a test; their values are fixed and do not vary with the pretest probability of a disease. An ideal diagnostic test should be both highly sensitive and highly specific. In this case, fine-needle aspiration has a high sensitivity and specificity; however, the NPV is a better parameter for addressing the patient's question (ie, how likely is she to not have thyroid cancer given her negative test result?); in this case her NPV might be low given her prior radiation exposure. (Choice E) Validity represents the accuracy of the test (ie, the test measures what it is supposed to measure). It does not depend on the pretest probability of the disease. Educational objective: Negative predictive value (NPV) is the probability of not having a disease when the test result is negative. The NPV will vary with the pretest probability of a disease. A patient with a high probability of having a disease will have a low NPV with a negative test, but a patient with a low probability of having a disease will have a high NPV with a negative test.

A 45-Year-Old Woman Comes to the Office with a Palpable