Coustan DR. Diagnosis of gestational diabetes. Scan J Clin Lab Invest. 2014;74(S244): S27-33
Following publication of this article, an error was identified on page 32 in the Question and answers section:
The correct Questions and answers are published below.
Questions for Dr Coustan
Q (Young): Which recommendations are being used in the USA?
A. (Coustan): In the USA the ACOG recommendations for gestational diabetes are most commonly used, although some centers have adopted the IADPSG recommendations.
Q (O’Shea): Regarding screening for gestational diabetes, you have spoken of the disparity between the USA and Europe, but we have disparity in approach in Ireland. There are several issues:
1)One problem is economic because, to have fasting, one hour and two hour specimens collected, the patients have to stay for long period in the clinic which can be difficult for them. Could we do just the one hour specimen, since the two hour value picks up only an additional 2 % of cases?
2) Could we do fasting glucose and HBA1c picking up at least 20 % of cases then screen the others fully?
3) Should there be universal screening or targeting of those at high risk?
A (Coustan): A single, fasting measurement is ideal for convenience but all three samples contribute independently to adverse outcomes and for this reason, all three are recommended. In different populations the various samples demonstrate greater or lesser contributions to the diagnosis, so center-specific approaches may be reasonable.
Regarding HbA1c, it is very useful in early pregnancy in pre-existing diabetes. It's not very sensitive for gestational diabetes.
Economic problems are a big issue in every country but the solution is not to ignore the problem but rather to strive for more efficient solutions.
Q (Kallner): When we make these recommendations we work with population means and medians but when we move to the real world we have to judge results from an individual patient: When you say 5.1 mmol/L as cut off for fasting glucose that translates into an individual result with an uncertainty. If you look at the German recommendation they allow an uncertainty of 11 % (95 % level) for glucose measurements. This means ± 1 mmol/L i.e. an uncertainty interval of 4.1-6.1 (k = 2) mmol/L. In a recent study we found an uncertainty of ± 3 % or 4.9-5.3 mmol/L (k = 2). The cut-off is not as distinct as you assume and decisions need to take the uncertainty into account.
A (Coustan): In the laboratory this is a problem not only for glucose. What is the alternative to using a cut-off? It depends on a good quality control in the laboratory. Whether the CV is 1 % or 5 %, there has to be a cut off somewhere.
Q (Kallner): As an alternative we may incorporate the uncertainty depending on what you want, to increase sensitivity or specificity.
A (Coustan): That is true for screening but not for diagnosis.
Q (Kallner): It could be applied also to diagnosis. Do we know if the cut-off has been set to optimize rule-in or rule-out?
Comment (Young): The test is defining the disease so it is hard to talk of the sensitivity and specificity of the test.
Comment (Lazzarotto): A screening test needs very high sensitivity, a priority for diagnostic tests is a high specificity.
Comment (Coustan): I don't agree because for a diagnostic test I would expect 100 % for both sensitivity and specificity since it defines the “disease.”