Tuesday, July 17, 2007
What is a patient to do? The available medical treatments for every condition or disease under the sun appears to be growing exponentially for those of us fortunate enough to live in a prosperous country such as the United States. Paradoxically, one of the causes for our skyrocketing health insurance costs is the continuing appearance of new procedures, tests, and drugs which are supposed to improve our health, but have the side effect of squeezing many Americans out of health insurance altogether.
Those of us who have the dilemma of choice about our health care should indeed be grateful, but the security of health insurance can be quickly overrun with confusion and stress about health care decisions, which are increasingly made by patients rather than doctors. In this age of ever new technologies and drugs, a primary care physician cannot hope to be up to speed on all the remedies available for all diseases and conditions, even those which do not require a specialist. Even doctors that try to keep up must sort through a medical literature that is sloppy and misrepresented.
An example of this is in prenatal care. An OB/GYN provides a dizzying array of tests and procedures for those concerned about the health of their unborn children, while often not having the time to fully understand the implications for your decision-making of the tests that they offer. The blood test known as the “triple test” or “triple screen,” seems now to be a prenatal standard. The test detects levels of three maternal blood components whose quantities vary in a way that correlates somewhat with the condition known as Down Syndrome (DS), in which a person has an extra copy of chromosome 21 (which causes many varied mental and physical impairments). The older the mother, the more likely that DS will occur (although a small proportion of cases do come from the father). For example, the risk of DS is roughly 1 in 365 for a 35-year-old mother, versus 1 in 120 for a 40-year-old mother (reports of these proportions vary depending on the data source).
In the best case scenario, the triple test will be offered with the doctor making clear that its variability results in both false positive results (5-10%), in which the test will indicate Down Syndrome when the baby is normal, and false negative results (10-15%), in which the test will indicate a normal baby when it actually does have DS. Unfortunately, most people - especially average patients, and often even the doctors themselves - do not know how to weigh these probabilities, and therefore do not truly understand what the test is telling them. When a patient has the triple screen done simply as a matter of routine, or because the doctor suggested it, but has not yet decided whether or not to undergo amniocentesis (the only definitive prenatal screen for several birth defects including DS), or whether or not to continue a pregnancy with an affected fetus, a false positive result can add a lot of stress to the already stressful condition of pregnancy itself.
Fortunately, one can calculate the various probabilities implied by the different possible test results using a probability tree, and make a truly informed decision about whether to have the test done or not. In the figure below are example calculations for both a 35-year-old and 40-year-old mother. Note that the numbers are probabilities, rather than percentages (.9975 = 99.75%). (Click to enlarge in a new window.)
Because the false positive rate is so high for this test (I used the conservative end of the scale for these calculations), every woman doing this test should decide before getting the results back what she will do if they are positive. If she is unwilling to do amniocentesis under any circumstances, she should not get this test because it could create undue stress if returning positive. If she is willing to do amniocentesis (bearing in mind that the procedure carries its own probabilities of miscarriage, from .5%-2%), she should decide if a negative result would be satisfying enough to her to then choose not to do amniocentesis. If certain knowledge is paramount above all, women would be wise not to waste time and money with the blood test, and simply go ahead with amniocentesis.
It is tempting of course to do the test to reassure oneself that everything is normal, but a negative result does not completely rule out Down Syndrome, although it greatly reduces the probability. Another caveat is to note that all of these probabilities are estimates. False negative and positive rates vary with exact stage of the pregnancy, and different data sources give slightly different probabilities for DS based on age.
In a perfect world, every obstetrician would have a chart in his or her office with the probabilities of a Down baby based on age and the positive or negative results of the triple screen. Based on my own anecdotal evidence, they do not, and are thus unable to fully advise patients on the implications of this test.
It is difficult to navigate the world of medical decisions these days, but a little research on potential tests and procedures is worth the time. Do not expect your doctor to anticipate all the implications of your test results, with any test. A doctor’s failure to do so, however, does not make him or her a bad doctor, just a busy person like you who does not have time to grasp the nuances of every medical treatment or test available.
Sphere: Related ContentTags: genetics, health, statistics
