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HbA1c Screening in Routine Clinical Practice

Posted: Sunday, January 31, 2010

Hemoglobin A1c (HbA1c) may be useful for diabetes screening and diagnosis in routine clinical practice, according to the results of a new study.

"With current screening tools (fasting plasma glucose [FPG] ± oral glucose tolerance test [OGTT]), the prevalence of undiagnosed diabetes remains high," write Zhong X. Lu, PhD, from Melbourne Pathology Services in Australia, and colleagues. "HbA1c provides a practical alternative for screening [that] is more convenient and reproducible than is blood glucose. As optimal cut-offs are still in debate, we here explore HbA1c levels that confidently 'rule-out' and 'rule-in' diabetes in two different Australian populations."

The goal of the study was to assess the value of HbA1c for screening and diagnosis of undiagnosed Type 2 diabetes, as determined by OGTT, in clinical and general populations. In a clinical group (MP population) of 2,494 persons, the prevalence of undiagnosed diabetes was 34.6%. This group was used to derive HbA1c cutoff values, which were 5.5% or more to "rule out" and at least 7.0% to "rule in" diabetes. These values were tested in a population-based sample (AusDiab) of 6,015 persons, in which the prevalence of undiagnosed diabetes was 4.6%.

Using an HbA1c cutoff value of 5.5% or less, sensitivity for diabetes was 98.7% in MP and 83.5% in AusDiab. With use of an HbA1c cutoff value of 7.0% or more, specificity was 98.2% for MP and 100% for AusDiab. In both populations, glucose status was abnormal in 61.9% to 69.3% of those with impaired HbA1c levels (5.6% - 6.9%).

"HbA1c <5.5% and >7.0% predicts absence or presence of Type 2 diabetes while at HbA1c 6.5-6.9%, diabetes is highly probable in clinical and population settings," the study authors write. "A high proportion of people with A1c [impaired A1c] have abnormal glucose status requiring follow-up."

Limitations of HbA1c as a screening/diagnostic tool include method bias, certain confounding medical conditions such as hemoglobinopathies and anemia, and cost.

"Although the cost of HbA1c is slightly higher than for FPG, the overall efficiency of using HbA1c as first line for diabetes screening may facilitate early diagnosis and reduce the health burden associated with diabetes complications," the study authors conclude. "Our study supports recommendations to use HbA1c for diabetes screening and diagnosis. Using two, rather than one, cut-off values for HbA1c achieved high sensitivity for screening plus optimal specificity for diabetes diagnosis." 

Source: Diabetes In Control: Diabetes Care. Online January 12, 2010.

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