Current standards of medical practice from the American Diabetes Association (ADA) (1) and the parallel guidelines from NACB (2) include relatively straightforward criteria for diagnosis of diabetes. Guidelines also include instructions and interpretations for the oral glucose tolerance test (OGTT), where needed for diagnosis of diabetes, and for a combined screening and diagnosis OGTT for gestational diabetes, which is recommended for all not-known-to-be-diabetic pregnancies at 24-28 weeks gestation (Table 1).
Despite the existence of these very up-to-date (2011) guidelines for gestational diabetes OGTTs, requests for gestational OGTTs at our institution come primarily from physicians who expect testing to occur according to 2001 guidelines of the American College of Obstetricians and Gynecologists (ACOG) (3). These comprise two separate tests for screening and diagnosis (Table 2).
In providing both ADA and ACOG OGTTs -- which are conducted primarily by outpatient phlebotomy services at our institution -- we thus require four separate procedures (ADA OGTT, ADA gestational OGTT, ACOG gestational OGTT screen, ACOG diagnostic gestational OGTT) and availability of three separate glucola drinks (50 g, 75 g, and 100 g). One disadvantage of the multiplicity of available tests is that it leads to a certain regular rate of misorders, with result of physicians not always obtaining results according to their desires and expectations.
The ACOG and ADA criteria for diagnosis of gestational diabetes are obviously not aligned. The need for two OGTTs to diagnose gestational diabetes according to ACOG criteria would appear to be a disadvantage for patients relative to the single test recommended via the ADA guidelines. Additionally, note that patient fasting is not required for the ACOG screening OGTT. That seems counterproductive to purpose -- a built-in, results-muddling preanalytical variable (4, 5). Considering the time and effort that a patient must spend for any appointment, my opinion is that this lack of control of the initial conditions of the test represents a less-than-optimum service to the patient.
I have posted on-line commentary at our institution concerning the differences between the ADA and ACOG tests and advocating use of the more modern and simpler guidelines from ADA. From the perspective of physicians ordering the ACOG test, however, they are already "with the program" within their own formally sanctioned standard of care. Yet the availability of different tests and criteria for diagnosis of gestational diabetes raises the general issue of non-uniformity of standard of care within our institution. However, this is not an area where the laboratory can dictate practice. It will be interesting to see whether revised ACOG guidelines will ultimately appear, and whether they will align with ADA.
Although a fasting glucose value may be sufficient to diagnose diabetes according to ADA, we have suspended the practice of evaluating fasting glucose prior to administration of the glucose load, for two reasons: 1. physicians usually want the test to proceed anyway (explanation: they are choosing to interpret results as if measurements within one OGTT are independent measurements), and 2. evaluation of fasting glucose prior to glucose load is not part of either the ACOG guidelines or the WHO procedure (6) to which ADA guidelines refer. It would seem unwise to pour glucose into someone who is patently diabetic upon fasting (e.g., fasting glucose > 200 mg/dL), but there are no formal guidelines that I am aware of concerning suspension of testing based on fasting measurement -- hyperglycemia is simply listed as a known risk factor for OGTT according to WHO. Should a glucose load have a fasting glucose measurement evaluated beforehand?
Last, having just left the AACC meeting in Atlanta, home of Coca-Cola: where does the ADA OGTT 75 g glucose stand relative to a Coke? A 0.5 L Coke contains 55 g of sugar (usually, mixed fructose and glucose; sometimes, sucrose). So, sugar content of an ADA OGTT is approximately equal to that of 0.7 L of Coke.