SLDCS is the largest population based study on diabetes conducted in Sri Lanka.
Patterns of FPG and OGTT results
Our female population had a higher overall post OGTT plasma glucose levels compared to males (p < 0.05). Relatively poor glucose handling was also seen in women in several other studies [16,17,18,19,20] but all involved women only in the middle age to elderly. Reason for this observation is uncertain. Longer gut half-life of glucose in women and inverse relationship between glucose tolerance and body height may at least partly explain this observation [16, 19]. Higher rates of IGT among women and rising OGT glucose values with age may contribute to higher risk of macro and microvascular complications of diabetes in this population. Currently practiced 2 h OGTT values for diagnosis of pre-diabetes and diabetes showed good sensitivity and specificity.
FPG was higher among men compared to women although the difference was statistically insignificant. Similar observations have been made in other studies [16,17,18,19,20]. Though the FPG reached a peak at 50–59 years, OGTT increased with aging. In contrast, studies in other countries have shown a significant increase in both FPG and post OGTT plasma glucose levels with aging [21,22,23] which is thought to be occurring partly due to reduced muscle mass and physical inactivity. This raises the need for the addition of post OGTT plasma glucose level for increasing the sensitivity for detection of diabetes in the elderly in our population.
FPG cut off for pre-diabetes/diabetes screening
According to our analysis, an FPG cut off value of 5.6 mmol/L had a good specificity (94.2%) for diagnosis of abnormal hyperglycemia (pre-diabetes and/or diabetes) but had a relatively poor sensitivity (43.8%). But for diagnosis of diabetes alone it had higher sensitivity (80.8%) and specificity (92.1%). Once the cut off value was lowered furthermore it improved the sensitivity at the expense of minor lowering of specificity. We identified an FPG level of 5.3 mmol/L as the most appropriate cut off value for our population in diagnosing both pre-diabetes and/or diabetes.
Lower cut off values for FPG were also recommended by other Asian countries ranging from 5.6–6.3 mmol/L for diagnosis of diabetes [13,14,15, 24]. However in contrast to our results for diagnosis of pre-diabetes, several other studies recommend a cut off value of 5.6 mmol/L as a good cut off with a sensitivity and specificity close to 100% [25, 26]. Nevertheless none of the above studies involved South Asian population and in the majority participants were > 35 years of age. What caused these differences in FPG levels in the Sri Lankan population is unclear.
We had several limitations in our study. First, only a single FPG or OGTT was used for defining the cases in our study as pre-diabetes or diabetes and a repeated confirmation test was not carried out. But we assume the impact of this to be very small due to the large number of participants. Second, the study did not include participants from North and East provinces. Data from more nationally representative sample as well as from other countries of the region will give more robust information about optimum cut points. Third, our study sample comprised of more women than men. However, no significant difference was observed in cut points for men and women (data not shown).