Fulminant type 1 diabetes (FT1D) could present diabetes ketoacidosis (DKA) at early onset. It is crucial to identify FT1D from DKA manifestations in time at clinical practice. This study was aimed at investigating whether the fulminant index (FI), encompassing plasma glucose (PG) to glycated haemoglobin (HbA1c) ratio (PG/HbA1c), serum potassium ion (K+ ) to HbA1c ratio (K+ /HbA1c) and serum sodium ion (Na+ ) multiplied by HbA1c (Na+ *HbA1c), is a feasible indicator for early FT1D diagnosis. A total of 78 subjects were enroled, including 40 FT1D patients and 38 non-FT1D patients with DKA. We utilised receiver operating characteristic (ROC) curve analysis to determine the FI cut-off values between FT1D and non-FT1D groups and examined efficacies of FI based on statistics. ROC curve analyses showed that the maximum Youden's index for PG/HbA1c bonding to a cut-off value of 4.389, with the sensitivity of 75.0% and specificity of 81.6% in identifying FT1D from DKA. And optimal K+ /HbA1c cut-off value was 0.728 with a sensitivity of 90.0% and specificity of 84.2%. For Na+ *HbA1c, the best cut-off value was 923.65, and its sensitivity and specificity were 85% and 73.7%, respectively. These results suggested FI could work as a valid and convenient indicator for differentiating FT1D from initial DKA patients. FI (K+ /HbA1c) presented the best efficacy as an independent index.