Hint: look for areas of uncertainty in testing, for example
A good quality publication will provide details of the accuracy of genetic tests.
In order to make an informed choice about testing, you need to know about the reliability and limitations of testing, including the possibility of incorrect or unclear test results. Very few genetic tests are 100% accurate. There are two main causes of test inaccuracy:
Test accuracy is usually described in terms of the ability of the test to detect true cases of disease and is based on data from the development of the test amongst people with the condition. If a genetic test is described as being 80% accurate, it means that 8 out of 10 people who have the mutation and are tested will be detected and receive a positive result. This is the rate of true positive test results, which is often described as test sensitivity. The remaining 2 out of 10 or 20% of cases receive an inaccurate false negative test result - these people have the mutation but it is not detected by the test.
Test accuracy may also include information about the detection rate amongst samples of people without the genetic mutation. This is particularly important information for developing reliable screening programmes. Information may be provided about the rate of true negative test results, which is the number of people who do not have the mutation and who test negative (often described as test specificity), or false positive results, which is the rate of positive test results in people who do not have the mutation.
The varied nature of genetic conditions and tests is associated with different levels of test accuracy. In some cases, repeated or additional testing may be necessary to confirm or clarify test results. Some forms of genetic mutation may be hard to find or are as yet unknown, and a conclusive result may not always be possible. Different laboratories using the same test can also produce conflicting results due to variations in skills, standards and equipment. Some test procedures are technically difficult and may fail even in the most skilled hands. Technical failures can be due to difficulties or errors occurring during collection and storage of blood or tissue samples, cell culture growths in the laboratory, and interpretation of DNA sequences and karyotypes.
These are all areas where scientific research is ongoing, and scientists are constantly working to improve the accuracy of genetic tests.
Inaccurate or unclear test results can have a significant physical and emotional impact. Rather than providing reassurance or clear answers, testing may actually increase the amount of uncertainty you have about your health. A false negative result may mean that early opportunities to treat a genetic condition or prevent it being passed on have been missed. False positive results may produce anxiety and lead to unnecessary treatment and life changes (e.g. not having children). In each case, a prolonged process of repeat testing, referral and consultation may be involved. An inconclusive test result may mean you have to live permanently with uncertainty about your status.
The availability of clear and honest information about these issues is important when deciding about testing, as it will help you understand what the test can and cannot tell you and what can go wrong during testing. If you decide to be tested, this information can help you understand and prepare for the results and any further testing that may be recommended.
Rate the question according to whether it provides clear information about the accuracy of all the genetic tests described. This may also be described as reliability or certainty. Note that this question cannot tell you whether the reported accuracy is true and does not recommend an acceptable level of accuracy or format for reporting accuracy (there are no currently agreed standards, but there is growing international collaboration to address these issues).
Guidelines for rating the question
We have devised simple examples here to demonstrate the quality criterion underlying Question 8. However, the information rated by this question should also have clear sources and referencing where appropriate as outlined in Question 16. For further guidance and examples, please refer to the section on Question 16
Here is some fictitious data from research about a new genetic screening test being offered nationally to all pregnant women. The text demonstrates how these data are used to provide information for the public about the test's accuracy. We have used a baseline figure of 1000 pregnancies to provide a simple numerical demonstration:
|Does the baby have the genetic mutation?|
|Number of pregnant women receiving a Positive Test Result||(a) 9 out of 10 (90%) are true positives||(c) 20 out of 990 (2%) are false positives||29 out of 1000 receive positive test results|
|Number of pregnant women receiving a Negative Test Result||(b) 1 out of 10 (10%) are false negatives||(d) 970 out of 990 (98%) are true negatives||971 out of 1000 receive negative test results|
|Totals||(e) 10 in 1000 (1%) babies have the mutation||990 in 1000 (99%) babies don't have the mutation||1000 pregnancies are tested|
This genetic condition affects approximately 1% or 10 in 1000 of all babies born every year (box e). A screening test for the condition is now offered to all pregnant women and consists of a simple blood test taken at the 12 week antenatal check. The test involves analysing the mother's blood sample in the laboratory for a marker of the genetic mutation that causes this condition.
The test is very accurate, picking up the majority of affected babies- 90% of cases in all health regions (box a). This means that 9 out of 10 babies with the condition are detected at the 12 week antenatal screening. However, the test cannot detect all forms of the genetic mutation, and this means that 10% of pregnancies producing a negative screening test result are actually affected - the screening test produces a 1 in 10 false negative rate (box b).
Due to problems with the interpretation of test results, there is also a chance that some women receiving a positive screening test result actually have babies who are not affected - around 2% of all pregnancies tested will give false positive results (box c). This still means that 98% of unaffected pregnancies will receive a negative test result (box d).
Counselling before the test will help you clarify these issues with your healthcare team and will enable you to work through the possible outcomes and implications of testing. If you receive a positive screening test result, you will be contacted for a repeat test to check the result and if this is also positive, you will offered a diagnostic test and further support. If you receive a negative test result you will also be able to discuss your results with the healthcare team. Full details of the test procedure, results, and some of the issues you may need to consider before and after testing are outlined later in this leaflet.
Some independent clinics offer testing earlier in pregnancy for a small fee. However, tests conducted before 12 weeks may be associated with higher rates of false-negative results. You should discuss these issues with your doctor or midwife if you are considering earlier testing.
A hospital oncology department handout provides the following information on a genetic test:
This test reliably detects the condition in 75 to 85 per cent of cases. However, it is associated with a false positive result (positive result when the condition is absent) in 3 to 10 per cent of cases. Accuracy is higher in public laboratories but waiting times for testing and results can be many months longer than other providers. If you decide to be tested at this clinic, we will take 2 blood samples so that we can check our results, and. you may be asked back to provide further samples if the test results are unclear. You will be called back for counselling and treatment if your results are positive. If you receive a negative or inconclusive test result, you will still be monitored regularly and recommended for further testing if you develop symptoms of this disease.
A hospital genetics department handout includes the following information:
This test is very accurate, detecting the majority of cases where there is a faulty gene.
Additional rating notes: Test accuracy is mentioned but not clearly explained.
A commercial online health encyclopaedia includes the following information about a genetic test:
It is important that you know that there is no method of detection or prevention that has been proven to be completely effective.
Additional rating notes: Test accuracy issues are implied but not clearlyexplained.
A maternity hospital leaflet for parents provides information on neonatal screening. It includes a brief description of the conditions being detected and the test procedure, which is a blood spot test. The conditions are described as rare but there is no information on prevalence or incidence. Some conditions are also described as inherited with no further explanation of how they are passed on, or whether the parents, older children or future pregnancies would also be affected if the test is positive. Screening is described as aiming to detect babies more likely to have these conditions, and results are described as either indicating your baby is normal, or thought to have a condition which may require further action. The possibility that a baby may be found to be a carrier for a condition is also mentioned without any explanation of this term. No further information or statistics are provided on each stage of testing.
Additional rating notes: The blood test is used to screen for a number of genetic conditions which would involve several different genetic tests with varying reliability. However, there is no mention of test accuracy anywhere in the publication.