When implementing a new testing technique, especially with qualitative and semi-quantitative tests, laboratory managers are often interested in the following indicators: Sensitivity and specificity of the testing method. When referring to Sensitivity and Specificity, there are two concepts: False Positive and False Negative.
1. SENSITIVITY AND SPECIFICITY
The sensitivity of a test is the proportion of cases that actually have the disease and have a positive test result out of all the cases that have the disease. The formula to calculate sensitivity is as follows:
Sensitivity = Number of true positives/(number of true positives + number of false negatives)
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The specificity of a test is the proportion of cases that are truly disease-free and have negative test results among all cases that do not have the disease. Specificity was calculated according to the following formula:
Specificity = Number of true negative cases/ (number of true negative cases + number of false positive cases)
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When referring to Sensitivity and Specificity, there are two concepts: false positive and false negative. So what are false positives and false negatives?
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2. WHAT ARE FALSE POSITIVES AND FALSE NEGATIVES?
a. A false positive is a positive test result when the person taking the test actually does not have the disease.
b. A false negative is a negative test result while the person taking the test actually has the disease.
Thus, false negative or false positive is a condition in which the test results obtained are not accurate for the disease condition.
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3. WHY THERE ARE FAKE POSITIVE AND FALSE NEGATIVE PHENOMENON
We all know that every process has vulnerabilities. Even the most accurate tests only give 99% accurate results.
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There are several causes of false positive and false negative test results. Let's see a few examples below:
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Medical examination, history taking, pre-test consultation to see when the patient gets sick? Did the patient comply with the regulations before taking samples for testing? This is extremely important because if not followed, it will lead to misleading results because there are a number of confounding factors.
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- The time of sampling and the patient's constitution/condition at the time of sampling are very important. For example: HIV testing during the window period will give false negative results because the body of the person doing the test is infected with the HIV virus but the body has not yet had time to create antibodies against the HIV virus, so when testing for negative results. A small number of cases gave false positive results. This means that the patient is not actually infected with HIV, but the result returned is positive. This case may be due to: mistakes during the testing process or because the patient during the test is suffering from diseases such as fibrosis, liver failure, tuberculosis.... or the patient is taking certain medications that affect the ability to identify HIV antibodies during testing.
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- Dengue fever has three stages: fever stage, shock stage, recovery stage. In the fever stage, it is difficult to recognize the disease. Furthermore, although there are early diagnostic tests for dengue fever, there are still cases of false negatives. The difference in blood collection time is one of the reasons leading to negative results but still dengue fever. When testing the blood count in the first 1-2 days, the amount of virus in the blood contains a lot, so some cases give negative results. Or the patient takes the test on day 5-6, when the virus in the blood has decreased, and the result can be negative.
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- Beta-HCG index in blood and urine plays an important role in diagnosing early pregnancy. The method of using a quick pregnancy test to find changes in beta-HCG concentration in urine gives fast and convenient results because this is a qualitative method (the test gives negative or positive results). The quantitative testing method for beta-HCG blood levels provides accurate results to help doctors diagnose pregnancy as well as screen for other related issues such as calculating gestational age and diagnosing abnormal pregnancies (ectopic pregnancies), diagnose the risk of miscarriage, and screen for Down syndrome. Although both methods give very accurate results and are widely applied, in reality (if the cause due to the quality of test strips or errors of the testing machine is excluded), there are many cases where the results are incorrect. False negative or false positive results for many different reasons. If the test is done too early, before the body has had time to produce enough beta-HCG, it can lead to false negative results. Because beta-HCG levels change very rapidly during early pregnancy, the quantitative beta-HCG test should be repeated within 48-72 hours to observe changes in this hormone level. Some medications such as diuretics, antihistamines, and sedatives can also cause false negative beta-HCG test results. If the test sample contains excess protein, blood or pituitary gonadotropin, it can lead to a false positive test result. This condition can also occur if the body produces antibodies that contain fragments of the HCG molecule.
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4. HOW TO DETECT FALSE POSITIVES AND FALSE NEGATIVES, AND HOW TO FIX IT
During clinical practice, the following principles must be followed:
- Check/compare with the patient's history, medical history, and clinical symptoms to see if the test results are appropriate (the doctor discusses the test results).
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- If not consistent, it is likely to be a false negative or false positive result.
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- When clinical symptoms/medical history/history do not match the test results, the clinician needs to discuss with the testing department to recheck the results, analyze the cause and, if necessary, consider test again. For example: If you are not sure about the HIV test results, retest after about 1-3 months for accurate results.
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- HBsAg test is the decisive test to diagnose hepatitis B. To conclude whether or not you have hepatitis B, you must do an HBsAg test, not an HBV DNA test. However, there are some cases of false positives, meaning the patient does not have hepatitis B but the test still gives a positive result. In this case, it is necessary to conduct a confirmatory test with a neutralization test or test with another standard test. For patients in this condition, they should see laboratory staff for advice and resolution. On the contrary, there are some patients with hepatitis B but the test results are negative. Those are false negative cases. The cause of false negatives may be due to technical errors or the use of low-sensitivity tests that are unable to detect patients with low HBsAg levels. In this case, it is necessary to retest with highly sensitive tests to get a final conclusion.
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6. HOW TO REDUCE THE RATE OF FAKE POSITIVE AND FAKE NEGATIVE?
- Laboratories select high quality chemicals/biological products (with high sensitivity and specificity).
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- Testing equipment needs regular maintenance/maintenance/calibration according to regulations.
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- Testing staff are trained to be proficient in performing standard testing procedures.
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- Patients should choose to be examined at reputable hospitals and can learn in advance about the accuracy of the test. In addition, patients should pay attention to following the instructions before the test, try to provide the most detailed and accurate information to the doctor, and absolutely should not lie for any reason.
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Source of article: https://www.vinmec.com/vi/tin-tuc/thong-tin-suc-khoe/suc-khoe-tong-quat/nao-la-duong-tinh-gia-va-am-tinh-gia/
