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How do practitioners test for gestational diabetes?
Testing for GD is a two-stage process. The first step is a screening test, which is generally administered to all pregnant women. The screening test is usually given somewhere between week 24 and 28. For this test, you may be asked to drink a glucose solution and have a blood sample drawn an hour later, or you may simply be asked to give a blood sample. If your blood glucose value exceeds a threshold amount, you will be asked to return for an Oral Glucose Tolerance Test (OGTT). The various protocols disagree on the amount of glucose and the threshold value (29).
For the OGTT, you will be asked to come in after fasting overnight. Blood will be drawn, you will be given a glucose solution to drink, and blood will be drawn one, two and three hours later. The glucose solution may make you nauseous. As with the screening test, the recommended amount of glucose and the diagnostic thresholds vary from protocol to protocol (29). Some guidelines only stipulate a fasting glucose and a two hour value (29).
What are the problems with gestational diabetes testing?
A diagnostic test should be reproducible, meaning you get the same results when you repeat the test. Thresholds should be values at which complications either first appear or incidence greatly increases; and normal ranges should apply to the population being tested. The OGTT is none of the above.
Obstetricians adopted data from the original 1950s studies as the normative curve for all pregnant women, but they shouldn’t have. For one thing, those researchers tested women without regard to length of gestation, whereas today, doctors typically test women at the beginning of the third trimester. Glucose values rise linearly throughout pregnancy, but no corrections have been made for this (15). For another, they studied a population that was sixty percent white and forty percent black. Hispanics, Native Americans and Asian women average higher blood sugars than black or white women (10,57). This means values for that 1950s population have been established as norms for all women, which in turn means that some women are being identified as diseased simply because of race.
The OGTT also isn’t reliable. When pregnant women undergo two OGTTs a week or so apart, individual test results disagree twenty to twenty-five percent of the time (5,23). A person's blood sugar values after ingesting glucose (or food) vary widely depending on many factors. For this reason, the OGTT has been abandoned as a diagnostic test for true diabetes in favor of high fasting glucose values, which show much greater consistency, or values after eating of 200 mg/dl or more, which are rare (46,52). Moreover, pregnancy compounds problems with reproducibility. Because glucose levels rise linearly throughout pregnancy, a woman could “pass” a test in gestational week 24 and “fail” it in week 28 (55). These same reproducibility problems hold true for the glucose screening test that precedes the OGTT (47,55).
More importantly, no threshold has ever been demonstrated for onset or marked increase in fetal complications below levels diagnostic of true diabetes. The original researchers chose their cutoffs for convenience in follow-up, but all studies since have used their criteria or some modification thereof as a threshold for pathology in the current pregnancy. Numerous studies since have documented that birth weights and other outcomes fail to correlate with the 1950s or anybody else's thresholds. Today’s researchers acknowledge that the risks of glucose intolerance almost certainly form a continuum and that screening and diagnostic thresholds are arbitrary (7,29-30,48,51).
Several organizational bodies that have looked critically at the GD research have come out against GD testing. A Guide to Effective Care in Pregnancy and Childbirth, the bible of evidence-based care, relegates screening for gestational diabetes to “Forms of Care Unlikely to be Beneficial (12).” The American College of Obstetricians and Gynecologists says no data support the benefits of screening (1). The U.S. Preventative Services Task Force and the Canadian Task Force on the Periodic Health Examination both conclude that there is insufficient evidence to justify universal GD screening (4,11).