Practical Classification Guidelines for Diabetes in patients treated with insulin: a cross-sectional study of the accuracy of diabetes diagnosis

Abstract

Background Differentiating between Type 1(T1D) and Type 2 diabetes(T2D) is fundamental for appropriate treatment and management of patients, but can be challenging, especially when patients are insulin-treated. UK Practical Classification Guidelines (using age at diagnosis and time to insulin treatment) were developed, but their accuracy has not been assessed.

Aim To assess the diagnostic accuracy of the UK guidelines against “gold-standard” definitions of T1D and T2D based on measured C-peptide levels.

Design & Setting 601 adults with insulin-treated diabetes and diabetes duration >5years were recruited in Exeter, Northampton & Leicestershire.

Method Baseline information and a home urine sample for urinary C-peptide creatinine ratio (UCPCR, a measure of endogenous insulin production) were collected. “Gold-standard” T1D was defined as continuous insulin treatment within 3 years of diagnosis and absolute insulin deficiency (UCPCR<0.2nmol/mmol >5years post-diagnosis); all other patients classed as T2D. Diagnostic performance of the clinical criteria assessed and other criteria explored using ROC curves.

Results UK guidelines correctly classified 86% of participants.

Most misclassifications occurred in patients classed as T1D who had significant endogenous insulin levels(57/601;9%); the majority in those diagnosed >35y and treated with insulin from diagnosis(37/66;56% misclassified).

Time to insulin and age at diagnosis performed best in predicting long-term endogenous insulin production(ROC AUC=0.904 and 0.871); BMI at diagnosis was a less strong predictor of diabetes type (AUC=0.824).

Conclusion Current UK guidelines provide a pragmatic clinical approach to classification that reflects long-term endogenous insulin production; caution is needed in older patients commencing insulin from diagnosis, where misclassification rates are increased.