Screening and diagnosis of type 2 diabetes with HbA_1c

Clinical question

Is hemoglobin A_1c (HbA_1c) testing appropriate for screening and diagnosis of type 2 diabetes mellitus?

Evidence

• Agreement between HbA_1c and fasting plasma glucose (FPG) or oral glucose tolerance testing (OGTT) is poor:

  • -25% to 27% agreement for HbA_1c and FPG^1,2;
  • -22% to 33% agreement for HbA_1c and OGTT.^1,3,4

• Some studies find HbA_1c (≥ 6.5%) would diagnose less diabetes than OGTT^1,5,6 (eg, HbA_1c missed 60% of the cases OGTT diagnosed⁶); some find HbA_1c (≥ 6.5%) would diagnose more diabetes than OGTT^2-5 (eg, OGTT missed 35% of the cases HbA_1c diagnosed⁴).

• In predicting outcomes of diabetes, HbA_1c

  • -performs as well as and often better than FPG^7-10 and
  • -might be similar to OGTT, but evidence is lacking^7,9; HbA_1c levels for best prediction vary by study.^7,10
• Using a diagnostic cutoff of HbA_1c ≥ 6.5%:

  • -
    Higher HbA_1c improves specificity; lower improves sensitivity.
  • -
    One study found HbA_1c of ≥ 6.5% had a sensitivity and specificity of 44% and 79%, respectively.¹¹
  • -
    While some data suggest the cutoff could be lower,^12-14 consistency is lacking,⁵ and racial differences do exist.¹⁵

Context

• Although FPG has been the preferred diagnostic test for diabetes for years, it requires patient compliance with fasting and has high intraindividual variability.^7,16

• Agreement between FPG and OGTT is also poor.^17,18

• HbA_1c is more expensive and not reliable in certain conditions (eg, hemoglobinopathies),¹⁹ but does not require fasting and has less intraindividual variability than FPG.⁷

• Recent American,²⁰ WHO,²¹ and Canadian Task Force on Preventive Health Care (CTFPHC)²² recommendations include HbA_1c of ≥ 6.5% for screening and diagnosis.

  • -Screening and diagnostic tests are the same in diabetes.²⁰
  • -Positive results (FPG, OGTT, or HbA_1c) should be confirmed with repeat testing using the same test.²⁰

Bottom line

An HbA_1c cutoff of ≥ 6.5% can be used to screen for and diagnose type 2 diabetes. Controversy persists around appropriate cutoffs and agreement with other tests.

Implementation

There is no evidence that screening adults at low or moderate risk of diabetes will improve outcomes; low-quality evidence suggests that screening high-risk adults could reduce complications.²² The CTFPHC recently published new guidance on screening for diabetes, identifying HbA_1c as the preferred test. The CTFPHC recommends using a validated risk calculator (preferably FINDRISC²³) to identify adults at high or very high risk. High-risk adults should be screened with HbA_1c every 3 to 5 years; those at very high risk should be screened annually. FINDRISC²³ requires consideration of diet, exercise, and body weight, so using these risk calculators offers opportunities to discuss other risk factors.

Tools for Practice articles in Canadian Family Physician (CFP) are adapted from articles published on the Alberta College of Family Physicians (ACFP) website, summarizing medical evidence with a focus on topical issues and practice-modifying information. The ACFP summaries and the series in CFP are coordinated by Dr G. Michael Allan, and the summaries are co-authored by at least 1 practising family physician and are peer reviewed. Feedback is welcome and can be sent to toolsforpractice@cfpc.ca. Archived articles are available on the ACFP website: www.acfp.ca.