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Oral Glucose Challenge vs HbA1c in Gestational Diabetes

Sep 14, 2019
 
Editor: David L. Joffe, BSPharm, CDE, FACA

Author: Kassey  James, Pharm.D. Candidate, LECOM School of Pharmacy

Is the Oral Glucose Challenge Test necessary in low-risk pregnant patients?

The use of HbA1c as a diagnostic tool for gestational diabetes has been minimal due to known physiological changes that occur during pregnancy. Dilutional anemia and the reduced half-life of red blood cells can result in lower HbA1c levels in pregnant versus non-pregnant patients. These issues have prevented the use of HbA1c as a gestational diabetes screening tool. Recent evidence is set to change this trend and support the use of HbA1c as a diagnostic tool in this patient population.

Oral Glucose Challenge Test is the current Gold Standard for diagnosing gestational diabetes but is inconvenient for patients due to the need to fast, lengthy exam time, and negative gastrointestinal side effects. Despite the physiological changes that can alter an HbA1c level in this population, this biomarker shows an accurate average of prior glycemic levels. HbA1c testing is easier for the patient without the requirement to fast and does not carry the unwanted side effects of nausea and vomiting seen in the Oral Glucose Challenge. Recently, The United States Preventative Task Forces released a review with four studies supporting the use of HbA1c as an appropriate screening tool for gestational diabetes.

A retrospective case-control study of 607 randomly selected pregnant patients between their 24th and 28th week of pregnancy explored the accuracy of using HbA1c as a gestational diabetes screening tool. Women with known diabetes mellitus, anemia, renal or pancreatic issues, or who were severely ill were excluded from this study. Mean differences were determined with a 95% confidence interval and the data was found to be statistically significant. The statistical analysis was performed using the Kolmogorov-Smirnov method and the t-test was used to determine mean HbA1c.

Results show that choosing a low HbA1c cutoff reduced false negatives and the number of patients required to complete the Oral Glucose Challenge. Based on this data, when using HbA1c as the initial test, if the level is below 5.45%, then the patient is considered normal. When the HbA1c result is between 5.45% and 5.95%, an Oral Glucose Challenge should be performed to rule out gestational diabetes. The patients diagnosed with gestational diabetes had a mean HbA1c of 5.7%, while the normal group was 5.34%. As predicted, HbA1c levels are consistently lower in the normal group, indicating this test is accurate. When comparing the sensitivity analysis of the two tests, HbA1c has higher specificity, resulting in fewer false positives. This method results in 85.7 % being correctly detected and only 2.8% of women being incorrectly labeled as having gestational diabetes.

Based on the results of this study and the review released by The United States Preventative Task Forces, the Oral Glucose Challenge should only be recommended in patients who do not meet the cut off from the HbA1c test. Using a low threshold for the HbA1c test between the 24th and 28th week of pregnancy prior to the Glucose Challenge Test is a simple, accurate, cost-effective, and patient-friendly diagnostic tool. Although guidelines have not been updated to reflect this data, evidence supports this as an appropriate screening tool. This study concluded that the Oral Glucose Challenge is an excessive measure in low-risk patients and recommends the inclusion of HbA1c as a diagnostic tool for gestational diabetes. At this time, HbA1c cannot replace the Oral Glucose Challenge Test, but when used in combination will significantly reduce the number of women burdened by the cumbersome oral test. Additional research is needed to confirm this data with a larger cohort from various geographical regions.

Practice Pearls:

  • A review from the United States Preventative Task Forces and this study support the use of HbA1c as an appropriate screening tool for gestational diabetes.
  • HbA1c cannot replace the Oral Glucose Challenge Test but can identify high-risk patients who qualify for the exam.
  • HbA1c has a higher specificity value than the Oral Glucose Challenge with fewer false positives.

Maesa, Jose-Maria, et al. “Screening for Gestational Diabetes Mellitus by Measuring Glycated Hemoglobin Can Reduce the Use of the Glucose Challenge Test.” Annals of Laboratory Medicine, vol. 39, no. 6, 2019, p. 524., DOI:10.3343/alm.2019.39.6.524.

Rajput, Rajesh, et al. “Utility of HbA1c for Diagnosis of Gestational Diabetes Mellitus.” Diabetes Research and Clinical Practice, vol. 98, no. 1, 2012, pp. 104–107., DOI:10.1016/j.diabres.2012.02.018.

Kassey James, Pharm.D.Candidate, LECOM School of Pharmacy