HbA1c test: fasting, home kits and how often

Fasting is needed for fasting plasma glucose, which requires at least 8 hours without calories [1]. If your clinician orders HbA1c together with fasting glucose or a lipid panel, follow the fasting instructions for those tests.

Because HbA1c is stable after the sample is drawn, it also avoids a common problem with glucose tests: glucose falls in a tube left unprocessed [1].

Standardization is what makes a result from one lab comparable with the next. The NGSP, launched in 1996, aligned US labs to the DCCT assay; by 2000, most certified methods were within 0.8% HbA1c of target [4]. Outside the US, many assays are IFCC certified [1].

On lab reports the test may appear as HbA1c, hemoglobin A1c, glycated hemoglobin or glycohemoglobin.

Labs separate glycated from non-glycated hemoglobin by methods such as chromatography or immunoassay. Some methods are affected by hemoglobin variants such as sickle cell trait, and ADA points to the NGSP list of assays with known interferences [1]. If you know you carry a variant, mention it when the test is ordered.

Home options include finger-prick analyzers that read a result in minutes and mail-in kits that send a dried or liquid sample to a lab. Accuracy depends on the device and on technique, such as getting a large enough blood drop.

Practical points:

• Use one method consistently to follow a trend.
• Check whether the device or lab is NGSP certified [1].
• Treat a home result in the prediabetes or diabetes range as a reason to get a lab test, not as a diagnosis [1].
• Remember that a kit cannot correct for the red cell factors that distort any A1c, such as iron deficiency or recent blood loss [5].

For people who want more than a number every few months, CGM shows glucose across the day and reports average glucose and time in range directly [6]. Its estimate of A1c, the GMI, often differs from a lab result [7].

Earlier screening applies to adults with overweight or obesity plus risk factors, and to people with other high-risk conditions [1]. NICE offers people at high risk a blood test at least once a year, preferably the same type each time [8].

Retesting sooner than about 3 months mostly shows the past, because HbA1c falls with a half-time of about 35 days [9].

Tell whoever orders the test about any of these. The hub's compiled table shows the direction and size of each effect. When HbA1c is unreliable, ADA advises plasma glucose for diagnosis and fructosamine or CGM for monitoring [1] [2].

ADA lists the trade-offs: glucose is cheap and widely available but varies with meals, stress and time of day; A1c is stable and standardized but more expensive and affected by red cell factors [1]. Using both from the same sample is one accepted way to confirm a diagnosis [1].

In ARIC, fasting glucose lost its link to cardiovascular disease once HbA1c was in the model, while HbA1c kept its link after adjusting for fasting glucose [10]. The oral glucose tolerance test remains the most sensitive way to catch early glucose problems, and ADA prefers it in some settings, such as after organ transplant [1].