How does your lab check fasting glucose when deciding whether to administer an oral glucose tolerance test (OGTT) to a patient?

A: In our hospital labs, before performing an OGTT, we test fasting plasma glucose using core laboratory chemistry analyzers. Reporting takes time because of all the steps involved, including collecting specimens, logging them in, centrifuging, and testing. However, this is not a problem for patients having OGTTs on-site. We make sure to test fasting specimens quickly, with the result handed immediately to a waiting lab assistant. If a result suggests diabetes, or gestational diabetes for pregnant patients, we don’t give the drink.

For patients at off-site specimen collection centers, we use a different process because specimens must be couriered to a testing site, which takes up to 4 hours. Prior to centrifuging and transporting a specimen, our collection center staff use a glucose meter to triage whole blood specimens and decide whether or not to give the drink. Even if the meter result does not support giving the drink, staff still send the separated specimen to our central lab for testing afterward so that there is always a fasting plasma glucose result available.

For nonpregnant patients, what criteria do you then use to determine if an OGTT is needed?

Most of our patients have OGTTs completed at off-site specimen collection centers that have glucose meters, so for nonpregnant patients, we set the threshold for denying the drink as the fasting plasma threshold for diabetes (≥7.0 mmol/L; 126 mg/dL) plus the analytical imprecision of the glucose meter (0.8 mmol/L; 14mg/dL). If the meter result for whole blood glucose is ≥7.9 mmol/L (142 mg/dL), we do not give the drink. For convenience, we use the same threshold when testing plasma glucose on core laboratory chemistry analyzers in our hospital labs.

How does your process differ for pregnant patients?

In Canada we follow the Diabetes Canada Clinical Practice Guidelines for screening for gestational diabetes, which recommend a two-step screening procedure. The first step of this procedure is a 1-hour plasma glucose test following a 50 g oral glucose load without prior fasting. Patients with results in an intermediate range (7.8-11.0 mmol/L; 140-198 mg/dL) go on to receive a 75 g OGTT.

The guidelines also state, however, that prior to giving the glucose drink during the OGTT, gestational diabetes can be diagnosed with a single elevated fasting glucose result (≥5.3 mmol/L; 95 mg/dL). At our hospital labs, we use this plasma glucose threshold to deny the drink.

At our off-site collection centers, we use a higher fasting whole blood glucose meter threshold of ≥6.1 mmol/L (110 mg/dL) to withhold the drink. This threshold has a sensitivity of 25% and specificity of 99.9% for a diagnostic fasting plasma glucose result (≥5.3 mmol/L; 95 mg/dL) on our meters and core laboratory chemistry analyzers.

What are the major pros and cons of using this approach?

A major pro is that we eliminate OGTTs in patients who probably don’t need them. This saves our lab and the healthcare system money, but most importantly it reduces patient discomfort and inconvenience. Glucose drinks are unpalatable to many, and may induce vomiting in pregnant women. The OGTT is also a 2-hour procedure with two additional blood draws, so canceling it allows patients to go home and avoid unnecessary phlebotomy.

The major con is that sometimes we might cancel the OGTT in patients who should have undergone it—especially at our specimen collection sites that use glucose meters. This may not be a major issue when screening nonpregnant patients for diabetes because asymptomatic patients must always be re-tested. However, it could delay treatment in pregnant patients because a single positive result can rule-in gestational diabetes. We therefore set a highly specific whole blood fasting glucose meter threshold for pregnant patients to ensure that our false-denial rate is exceedingly low (approximately three patients per year).

If you’re interested in learning more about our lab’s approach to this issue, read our study in The Journal of Applied Laboratory Medicine (J Appl Lab Med 2018; doi: 10.1373/jalm.2018.026047).

Lawrence de Koning, PhD, DABCC, FADLM, FCACB, is an associate professor with the University of Calgary and a clinical biochemist with Alberta Public Laboratories at Alberta Children’s Hospital in Calgary, Alberta, Canada. +Email: Lawrence.dekoning@alberta