Patient result monitoring of HbA_1c shows small seasonal variations and steady decrease over more than 10 years

Internal and external quality assurance materials often use highly processed matrixes. This can render the materials non-commutable. Monitoring laboratory methods with patient medians helps in identifying and correcting systematic errors that may affect diagnostic accuracy. The aim of the present study was to use HbA_1c patient results for monitoring of method performance over time.

Test HbA_1c results from 2010 to 2022 was analyzed (n=722,553) regarding changes over time and seasonal variation. The HbA_1c testing was initially performed on a Cobas 501 instrument using immunological detection but in May 2017 the method was replaced by capillary electrophoresis on Capillarys 3 Tera.

There was a steady decrease in HbA_1c values. From 2011 to 2021 the decrease was for 0.10 percentile 6.6 %, lower quartile 7.9 %, median 10.2 %, mean values 9 %, upper quartile 11.2 %, and 0.90 percentile 9.3 %. No clear shift in HbA_1c levels was observed due to the shift in methods. The median HbA_1c values per month was approximately 44 mmol/mol (6.2 %, DCCT/NGSP). The only month with a median HbA_1c that differed by more than 1 mmol/mol was July with a median value of 42 mmol/mol (6.0 %).

The patient data showed a similar decrease as in the National Diabetes Register which indicates that the method is stable over time without any sudden changes and that the seasonal variation is low. The continuous decrease in HbA_1c values over time is most likely to a shift towards earlier detection of patient with diabetes and improved treatment.