Single-Sample Confirmatory Test Ok for Diabetes Diagnosis

A single blood sample can be used to test for both fasting glucose and hemoglobin A1c (HbA1c) levels in order to identify undiagnosed diabetes in the population, according to a study published in the Annals of Internal Medicine. This confirmatory definition, the authors say, has high positive predictive value for future risk for diagnosed diabetes and is associated with risk for major clinical outcomes.

Current clinical definitions of diabetes often require repeated testing to confirm elevated levels of glucose or HbA1c in order to reduce the possibility of a false-positive diagnosis.

"Using a repeated test on a new sample from a subsequent visit to confirm diabetes can be logistically cumbersome, inconvenient, and expensive and can delay patient care," writes K.M. Venkat Narayan, MD, from Emory University in Atlanta in an editorial accompanying the study. "Whether two tests from the same blood sample can be used for both screening and confirmation is an intriguing question with practical relevance."

In the present study researchers analyzed 25-year data from 12,268 participants of the Atherosclerosis Risk in Communities) study without diagnosed diabetes at baseline. Confirmed, undiagnosed diabetes was defined as elevated levels of fasting glucose (≥7.0 mmol/L and HbA1c (≥6.5%) from a single blood sample. Unconfirmed undiagnosed diabetes was defined as no diagnosis and only one elevated measured.

The researchers found that at baseline (1990 to 1992), 978 participants had elevated levels of fasting glucose or HbA1c. Among these, 39 percent had both measures elevated (confirmed undiagnosed diabetes), while 61 percent had only one elevated measure (unconfirmed undiagnosed diabetes).

For identifying diabetes cases diagnosed during the first five years of follow-up, the confirmatory definition had moderate sensitivity (54.9 percent) but high specificity (98.1 percent). Specificity increased to 99.6 percent by 15 years, with a 15-year positive predictive value of 88.7 percent versus 71.1 percent for unconfirmed cases.

"This study provides construct validity for a confirmatory definition of undiagnosed diabetes that is based on a combination of HbA1c and fasting glucose measured in a single blood sample," write the authors led by Elizabeth Selvin, Ph.D. from Johns Hopkins University in Baltimore, Md. "Our data suggest high general concordance between fasting glucose and HbA1c levels; therefore, attention should be paid to any sizeable discordance between them because this may indicate a sample processing problem or a coexisting medical condition that may be interfering with either test."

Takeaway: Simplifying methods to screen for and confirm diabetes can facilitate clinical care of the individual and can benefit population-level efforts to identify diabetes at an earlier stage.