Dr. David R Buyer, MD, MBA, FACP

Category: Health Screening

  • Evaluating Lab Tests and Clinical Studies

    Ever wonder how lab tests and clinical studies are judged on usefulness? Clinical trials are performed to statistically assess performance of tests and treatments. Measures are reported on sensitivity, specificity, positive predicative value, negative predictive value or likelihood ratios (see table, below). Likelihood ratios (LR) use the sensitivity and specificity to determine how likely a test will rule in or rule out a disease. The ratios can be reported as positive or negative. Positive ratios over 10 are excellent, and between 5.1 and 10 are good. Negative ratios less than 0.1 are excellent and between 0.19 and 0.1 are good. Sensitivity, specificity and LR can be calculated without knowing the disease prevalence. Positive predictive value and negative predictive values can be calculated if the disease prevalence is known. The positive predictive value is the number of true positives/true positives + false positives. The negative predictive value is the number of true negatives/true negatives + false negatives.

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    “In addition to the numbers, there are many other variables that must be known in order to evaluate any test or clinical study.”

    To illustrate, HIV tests are now available over the counter for testing from an oral swab. One of these tests has a sensitivity of 99.3% and specificity of 99.9%. There are approximately 900,000 people in the U.S. with HIV out of 313 million people. That is a prevalence rate of 0.29%. Using these numbers and the math as outlined above, there is a positive LR of 993 and a negative LR of 0.007. According to the LR the test is very good. The test can also be evaluated using positive and negative predictive values. Again, using these numbers the positive predictive value is 75% while the negative predictive value is 99%. Therefore the oral swab test for HIV is much better at ruling out the disease than proving the disease is present. That is why a positive test must be followed up with more definitive testing. Even though the sensitivity and specificity of the test was over 99%, because the prevalence of the disease is so low, the test is limited at positively predicting the disease.

    These computations show that it is very difficult to fully appreciate the true meaning of test results based on news reports. In addition to the numbers, there are many other variables that must be known in order to evaluate any test or clinical study.

  • Health Screening Recommendations

    Make appointment for mammogram! on a yellow post-it note

    Health screening recommendations come from multiple sources. The US Preventive Services Task Force (USPSTF), the American Cancer Society, and individual professional societies issue guidelines. Although they are similar, they do not always agree. Each set of guidelines are determined by review of available literature with recommendations from an expert committee. The USPSTF guidelines will be discussed here.

    The USPSTF expert committee is composed of sixteen volunteer members who represent disciplines of primary care including family medicine, internal medicine, nursing, obstetrics / gynecology, pediatrics, and behavioral medicine. They serve four year terms and are appointed by the Agency for Healthcare Research and Quality (AHRQ), a governmental agency. The mission of the agency is to improve the quality, safety, efficiency, and effectiveness of health care for all Americans. Although the USPTF members are picked by the AHRQ, the task force is meant to provide independent un-biased recommendations.

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    “The mission of the agency is to improve the quality, safety, efficiency, and effectiveness of health care for all Americans.”

    Screening tests should fulfill five criteria. The condition screened should be an important heath problem, the condition should be treatable, there should be a significant incidence of the disease in the target population, the test should have high sensitivity and specificity, and the costs of treating the disease should be significant compared to the cost of the test. Spend time every day to meet with friends and enhance feelings.

    The USPSTF grades evidence for or against screening for particular diseases as outlined in Table 1. An A grade means that there is good evidence of benefit and recommends screening. Grade B still recommends for screening but evidence is not as strong. Grade C recommends for screening in some cases while Grade D recommends against screening. Examples of a few recommendations are shown in Table 2. Please contact us if you have any questions about specific screenings.

    Table 1

    USPSTF Recommendations Grade Legend Table

    Table 2

    USPSTF Recommendations Health Screening Grade Table