Disposition index

The disposition index can be obtained on the basis of data that provide information on insulin sensitivity and beta cell function. Suitable sources include:

• Hyperglycemic glucose clamp investigations^[6]
• Frequently-sampled intravenous glucose tolerance testing (IVGTT or FSIGT) ^[1]
• Frequently-sampled oral glucose tolerance testing (OGTT or FSOGT)^[7][8]
• Simultaneous fasting measurements of insulin and glucose in conjunction with mathematical modelling (SPINA)^[9]

If clamp investigations are used the disposition index is defined as the product of the area under the insulin response curve (${\displaystyle AUC_{\Delta Insulin}}$ ) and the insulin sensitivity index (ISI_Clamp, average glucose infusion rate divided by average insulin concentration) with

${\displaystyle DI_{Clamp}=AUC_{\Delta Insulin}\cdot ISI_{Clamp}}$ .^[6]

Determining the disposition index on the basis of an FSIGT requires fitting the timeseries of insulin and glucose concentrations to the minimal model of insulin-glucose homeostasis.^[10] The disposition index is then calculated as

${\displaystyle {DI}_{FSIGT}={\varphi }_{1}\cdot {S}_{I}}$ 

from the first phase response of plasma insulin to the glucose injection (${\displaystyle {\varphi }_{1}}$ ) and the insulin sensitivity index (S_I).^[10]

Based on an oral glucose tolerance test a disposition index can be calculated with

${\displaystyle {DI}_{FSOGT}=IGI\cdot ISI_{composite}}$ 

from the insulinogenic index (IGI) and the insulin sensitivity index (ISI_composite).^[7][8]

The fasting-based disposition index (SPINA-DI) can be obtained from the product of the secretory capacity of pancreatic beta cells (${\displaystyle G_{\beta }}$  or SPINA-GBeta) times the insulin receptor gain (${\displaystyle G_{R}}$  or SPINA-GR):

${\displaystyle DI_{f}=G_{\beta }\cdot G_{R}}$ .^[9]

The four approaches deliver slightly different information. Although the results of clamp-, IVGTT-, OGTT- and SPINA-derived disposition indices significantly correlate with each other the correlations are only modest.^[11][12] In direct comparison, the SPINA-based disposition index (SPINA-DI) had higher discriminatory power for the diagnosis of diabetes than the OGTT-based disposition index according to Matsuda and DeFronzo.^[9]

Disposition index is used as a measure of beta cell function and the ability of the body to dispose of a glucose load. Thus a lowering of disposition index predicts the conversion of insulin resistance to diabetes mellitus type 2.^[13] Disposition index, but not insulin resistance, can predict type 2 diabetes in persons with normal blood glucose levels, but who do not have a family history (genetic predisposition) to type 2 diabetes.^[14]

Disposition index can be increased by aerobic exercise, but only to the extent that insulin sensitivity is improved.^[15]

• SPINA-GBeta
• SPINA-GR