Patterns of gestational diabetes diagnosis inside and outside of clinical guidelines

Hospital discharge codes are often used to determine the incidence of gestational diabetes mellitus (GDM) at state and national levels. Previous studies demonstrate substantial variability in the accuracy of GDM reporting, and rarely report how the GDM was diagnosed. Our aim was to identify deliveries coded as gestational diabetes, and then to determine how the diagnosis was assigned and whether the diagnosis followed established guidelines.

We identified which deliveries were coded at discharge as complicated by GDM at the Brigham and Women's Hospital in Boston, MA for the year 2010. We reviewed medical records to determine whether the codes were appropriately assigned.

Of 7883 deliveries, coding for GDM was assigned with 98% accuracy. We identified 362 cases assigned GDM delivery codes, of which 210 (58%) had oral glucose tolerance test (OGTT) results available meeting established criteria. We determined that 126 cases (34%) received a GDM delivery code due to a clinician diagnosis documented in the medical record, without an OGTT result meeting established guidelines for GDM diagnosis. We identified only 15 cases (4%) that were coding errors.

Thirty four percent of women assigned GDM delivery codes at discharge had a medical record diagnosis of GDM but did not meet OGTT criteria for GDM by established guidelines. Although many of these patients may have met guidelines if guideline-based testing had been conducted, our findings suggest that clinician diagnosis outside of published guidelines may be common. There are many ramifications of this approach to diagnosis, including affecting population-level statistics of GDM prevalence and the potential impact on some women who may be diagnosed with GDM erroneously.