1. FAMILY MEDICINE AND LABORATORY TESTS Elham
FAMILY MEDICINE AND LABORATORY TESTS Elham.Atabati Assistant professor of Rheumatology Birjand University of Medical Sciences

2. Clinical decision making using laboratory testing is base on the assumption that a given test is accurate and precise.

3. Diagnostic test accuracy is the ability of a test to distinguish patients with a disease from those who are disease free. Precision is a measure of the reproducibility of a test measurement when the same specimen is rechecked under the same circumstances. Sources of imprecision include biologic variability and analytic variability.

4. Biologic variability is the variation in a test result in the same person at different times because of physiologic processes, constitutional factors, and extrinsic factors .(Mc Clatchey, 2002)

5. Analytic variation refers to the variation in repeated tests on the same specimen and relates to analytic technique and specimen processing. With current technology, biologic variation plays a larger role than analytic variation in most laboratory test.

6. The current standard of comparison for laboratory results is the reference range, which is frequently defined by results that are between chosen percentiles (typically the 2.5th to 97.5th percentiles) in a healthy reference population.

7. Differences in gender , age distribution, race, ethnicity, or the setting (hospitalized vs ambulatory patients) between the reference population and the person receiving the test may be present. The person being tested should be tested under similar physiologic conditions (e.g., fasting, sitting, resting) as the reference population.

8. Reference ranges for a particular test can be the manufacturer’s suggested reference range or may be modified because of differences in the population using the laboratory. the normal or reference ranges must be made available to the ordering physician.

9. When selecting a reference range that includes 95% of the test results, 5% of the population will fall outside the reference range for a single test. When more than one test is ordered, the probability increases that at least one result will be outside the reference range.

10. discrepancies between a test’s efficacy and its effectiveness are common. Efficacy is a test’s performance under ideal conditions, whereas effectiveness is its performance under usual circumstances. A cutoff value may be chosen to separate “normal” from abnormal.

11. The optimal cutoff depends on the purpose of the test and essentially is a risk/benefit analysis. If disease exclusion is the goal, sensitivity and negative predictive value need to be maximized. It is important that negative results be true negatives as opposed to false negatives, so that a negative test has correctly excluded the individual as having disease

12. If disease confirmation is the goal, specificity and positive predictive value are critical. It is important that positive results are true positives and not false positives, so that healthy persons are not misidentified, especially when treatments (e.g., surgery) have serious risks.

13. High Sensivity+Negative test=Rule out
High Specificity+Positive test=Rule in

14. The predictive value of a test is directly related to the pretest probability of disease. When the prevalence of disease is high in the population, a positive test result is expected and a negative result is not expected, because the disease is common. Similarly, when the prevalence is low, a negative test result is anticipated because few people have the disease. These characteristics of predictive value become clinically useful when one compares the outcome of a positive or negative test result with the pretest probability of disease

15. Consider the example in which two tests are available for the diagnosis of a disease. Three combinations can lead to an affirmative diagnosis: 1. If one of the two tests is positive, the diagnosis is made. 2. A positive result for both tests is required before the diagnosis is confirmed. 3. The second test is performed only if the first is positive, and the person is labeled as diseased only if the second is also positive.

16. What actions will be taken, based on the test results
What actions will be taken, based on the test results? What are the expected benefits or harms that might occur, based on a positive or a negative result? Will ordering a particular test be more likely to help than harm the patient? Unfortunately, at present, randomized controlled trials (RCTs) that examine the outcomes of test-and-treatment strategies are not available for most clinical situations. Thoughtful systematic reviews of diagnostic test accuracy, linked with clinical evidence examining treatment options, may be the best available evidence to help guide decisions on diagnostic testing (Cornell et al., 2008).

21. The survey was based on live discussions with PCPs in two focus groups in different cities. It had five domains: (1) uncertainty regarding ordering and interpreting tests, (2) tactics to overcome uncertainty, (3) factors that influence test ordering, (4) challenges to appropriately order and interpret tests, and (5) perceived solutions. The results described here come from the responses of almost 1,800 PCPs who returned the survey, about 50% family physicians and 50% general internal medicine physicians

22. Responding physicians ordered diagnostic laboratory tests for approximately 31% of patient encounters (approximately 25 times a week). They reported being uncertain about which test(s) to order for 14.7% of patients who needed diagnostic laboratory tests and uncertainty about interpreting the results for 8.3%.

23. Among the top factors described by the PCPs as reasons to challenge the choice of laboratory tests were cost, including that to the patient; mandates from insurance companies to use specific laboratories; and the lack of available cost comparison data from the various laboratories. Perhaps most relevant to this audience is how PCPs deal with uncertainties when ordering laboratory tests.

24. In fact, PCPs told us that their interactions with the laboratory were most useful when they dealt with technical issues or location of missing results—not clinically relevant issues such as interpretation of test results!

26. Non modifiable physician factors : These included practice location, practice setting, age and sex, and specialization of the physician

27. Hospital based physicians ordered more tests than other practitioners, academic physicians ordered more tests and nonacademic practice physicians had a higher threshold of test ordering, the number of tests ordered by emergency room physicians did not differ from that by primary care physicians, and a health-maintenance organization setting did not affect test ordering.

28. female physicians tended to order more tests, give more referrals, and adhere better to guidelines than their male counterparts.

29. However, although specialists ordered more tests, they also ordered more focused tests and tests that were more likely to have positive results than internists

30. Modifiable physician factors: physician experience and knowledge, belief systems, fear of malpractice lawsuit and physician regret, financial incentives , awareness of the cost of testing, and education and feedback.