In recent years, an increasing number of FT1D cases have been reported in China, which has expanded our knowledge about the disease3, 13, 14. In clinical practice, DKA is one of the most commonly encountered lethal complications, nevertheless, for many reasons, physicians may not be able to obtain the patients’ previous diabetic histories in the first place. Since C-peptide level assessment could be time-assuming and could be not applicable under certain circumstances, prompt diagnosis of FT1D based on C-peptide could be impossible at this time. Alternatively, taking advantage of other accessible biomedical indexes would be an ideal solution for the timely identification of FT1D. Compared to autoimmune type 1 patients, FT1D patients showed lower levels of Na+ and HbA1c and significantly higher levels of K + and PG3, 5. In the present study, we found the same deviations between FT1D and non-FT1D DKA patients.
Hyponatremia and hyperkalemia are commonly found in the DKA, and the serum electrolyte changes are more extensive in the FT1D patients. There is a negative correlation between Na+ levels and PG and, conversely, a positive correlation between K+ levels and PG. The changes are more evident in patients with insulin-dependent diabetes mellitus than in those with non–insulin-dependent diabetes mellitus. The underlying pathophysiology mechanisms may include the movement of electrolytes between intra- and extracellular spaces, impaired insulin action, as well as hyperosmolality15, 16. Insulin activates Na+/K+-ATPase17–19. The activity of Na+/, K+-ATPase could be attenuated in insulin-dependent diabetic patients whose insulin secretion is impaired. In FT1D, hyponatremia and hyperkalemia could arise as a consequence of remarkedly increase of plasma glucose and devastation of insulin-producing capacity5, 13. Both endogenous or exogenous insulin is capable of effecting serum electrolyte levels, especially K+. Thus, DKA patients with insulin-dependent diabetes may not necessarily generate high serum K+ is they received insulin therapy. Similarly, for FT1D patients, those who have been treated with insulin may experience alleviation of hyperkalemia, but the PG or serum Na + may not respond as good if the patient was severely dehydrated.
Improvements in technology now permit the prompt testing of HbA1c. PG, Na+, and K+ are routine blood examination results. Therefore, FI (PG/HbA1c, K+/HbA1c, Na+*HbA1c) can be easily measured in clinical practice. Our results indicate FI (PG/HbA1c ≥ 4.39 mmol/L/%, K+/HbA1c ≥ 0.85 mmol/L/%, Na+*HbA1c ≤ 923.65 mmol*%/L) can be adopted as a new set of biomedical indexes in diagnosing FT1D. In recent findings on the diagnosis of FT1D, GA20, 1,5-anhydroglucitol (1,5-AG)/GA21, the GA/HbA1c ratio22 and the PG/HbA1c ratio23 were used for screening FT1D. The GA/HbA1c ratio and PG/HbA1c ratio are both FI.
Compared with those of acute-onset autoimmune type 1 diabetes mellitus (T1ADM) patients, both HbA1c and GA were significantly lower in FT1D patients. In the differential diagnosis between FT1D and T1ADM, ROC analysis showed that the optimum cut-off value for GA was 33.5% with a sensitivity and specificity of 97.4% and 96.8%, respectively20. And GA and 1,5-AG are indicators that reflect short-term glucose levels, 1,5-AG/GA can help facilitate the early differential diagnosis of FT1DM and T1ADM when HbA1c < 8:7%, with an optimal cut-off point of 0.321. FI (GA/HbA1c) for FT1D patients has already been reported to be higher than for patients with T2DM. ROC analyses showed that although both the specificity and sensitivity of HbA1c and those of serum GA for differentiating FT1D and T2DM were low, a cut-off value of 3.2 for FI (GA/HbA1c) yielded 97% sensitivity and 98% specificity for differentiating FT1D from T2DM. However, no significant differences in FI (GA/HbA1c) between T1ADM patients and FT1D patients were observed22. According to reported cases of FT1D in China, few primary hospitals in China monitor GA and 1,5-AG. Compared with HbA1c, GA 1,5-AG are not widely used in China. Liu L et al proposed a cutoff value of 4.2 for FI (PG/HbA1c), yielding 94% sensitivity and 98% specificity in differentiating FT1D from DKA23. However, that study did not limit HbA1c in the DKA group because if HbA1c exceeded 8.7%, we could rule out FT1D without the PG/HbA1c. Therefore, our study established a limit for the HbA1c level in the DKA group and compared it with the level in the FT1D group to test whether the PG/HbA1c ratio was effective and to explore whether there were better indicators for diagnosis.
As shown in the above figure and table, FI (PG/HbA1c) can differentiate FT1D from DKA. ROC analyses showed that the highest Youden’s index for FI (PG/HbA1c) was a cutoff value of 4.39, with a corresponding sensitivity of 75.0% and specificity of 77.8% in identifying FT1D from DKA (AUC: 0.818). FI (PG/HbA1c) at a cutoff value of 4.39 mmol/L/% among DKA patients was the best predictor of FT1D in China. Since K+ and PG were both higher in FT1D than in DKA, we used the ratio of these two parameters to HbA1c to construct the FI. Na+ and HbA1c were lower in FT1D than in DKA, so we constructed the FI by multiplying both to increase the difference between the two diseases.
K+/HbA1c and Na+*HbA1c were additionally constructed FIs. ROC analyses showed that the highest Youden’s index for FI (K+/HbA1c) was a cut-off value of 0.85, with a corresponding sensitivity of 77.5% and specificity of 94.4% in identifying FT1D from DKA (AUC: 0.899). FI (K+/HbA1c) at a cutoff value of 0.85 mmol/L/% among DKA patients was the best predictor of FT1D in China. ROC analyses showed that the highest Youden’s index for FI (Na+*HbA1c) was a cut-off value of 923.65, with a corresponding sensitivity of 85.0% and specificity of 69.4% in identifying FT1D from DKA (AUC: 0.814). FI (Na*HbA1c) at a cutoff value of 923.65 mmol/L/% among DKA patients was the best predictor of FT1D in China. We can also construct other FIs based on the differences in the data for the two diseases, but they are too complex and no better than these. The K/HbA1c ratio is the best FI for predicting FT1D from DKA according to the AUC (0.899). Since insulin can reduce K+, it is best to use K+ to create the FI before insulin intervention to more accurately reflect the real situation of patients.
Improvements in technology now permit the rapid testing of HbA1c. PG, Na+, and K + are routine blood examination results. Therefore, FI (PG/HbA1c, K+/HbA1c, Na+*HbA1c) can function as a simple tool that may be useful to identify FT1D in DKA patients. FI (PG/HbA1c ≥ 4.39 mmol/L/%, K+/HbA1c ≥ 0.85 mmol/L/%, Na+*HbA1c ≤ 923.65 mmol*%/L) can be adopted as a new clinical parameter in diagnosing FT1D.As mentioned in previous paragraphs, early diagnosis of FT1D could set alert for high-risk cases, or direct appropriate clinical intervention, and may help early management of drug-induced FT1D or pregnancy-associated FT1D cases.
Our study proposed a more convenient quantitative tool to help distinguish FT1D from DKA secondary to other types of diabetes. This tool could achieve early screening of FT1D prior to C-peptide assessment and DKA correction. Besides these advantages, there are still limitations in this study. Limited by the rareness of FT1D cases, our sample size is relatively small. An increase of observations on FT1D cases would provide more abundant data to verify the capacity of FI as a diagnostic index in the future. Also, our study focuses on the Chinese FT1D population, while how FI works in the population of other ethnicities remains unclear. Besides, our analysis did not involve other clinical factors that could also be susceptible to serum electrolyte, such as insulin usage and diarrhea of the patient. Therefore, if physicians apply FI in clinic, we suggest taking the effects of diarrhea and insulin usage into additional consideration.