The different standard methods used in measurement of insulin resistance include the hyperinsulinaemic euglycaemic (HEC) clamp, frequently sampled intravenous glucose tolerance (FSIGT) test minimal model analysis and insulin suppression test. Other hormonal based methods include homeostasis model assessment models, quantitative insulin sensitivity check index model, fasting insulin, fasting c-peptide and fasting glucose to insulin ratio.
2.5.1 HYPERINSULINAEMIC EUGLYCAEMIC CLAMP (HEC).
The HEC clamp is referred to as the gold standard for assessing insulin sensitivity/resistance.3 It allows researchers to quantify βeta cell sensitivity to glucose and the sensitivity of body tissues (muscle, fat, and liver) to insulin.
Procedure: The HEC is performed in the morning following a 12 hour fast. The procedure for the clamp begins with the insertion of two intravenous lines. One is placed in an antecubital vein for glucose and insulin infusion. A second is placed in retrograde fashion in a hand vein while the forearm is placed in a heated box. This line is used for drawing samples for glucose
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determination. The goal of the insulin infusion is to raise the plasma insulin concentration to a plateau approximately 100μU/ml of plasma over basal insulin levels to approximate usual postprandial levels and to maintain it at that plateau for a period of two to four hours. The objective is to raise insulin to a level that will suppress hepatic glucose production. To maintain this plateau and to keep the individual euglycaemic, variable amounts of 20% dextrose will be infused. Insulin dosage is at a fixed level and the amount of dextrose infused will depend on the subject’s insulin sensitivity. Maintaining plasma glucose concentrations at approximately 90 ± 2 mg/dl (euglycaemia) requires frequent blood sampling every 5-15 min for plasma glucose concentrations, which are measured with a precise glucose analyser. Samples for insulin levels are obtained every 30 to 60 min throughout the clamp procedure. The results of the test are then analysed by computing M values for each 20 minute interval of the test. A value of M reflects the amount of glucose metabolized and is reported in values of mg/ kg/ min.3
To determine insulin sensitivity, M/I ratio is calculated, where I is the plasma insulin response, and an M/I ratio is a measure of the quantity of glucose metabolized per unit of insulin concentration.3 If an individual is relatively insulin sensitive, larger amounts of glucose will need to be infused for a given amount of insulin to keep the individual’s blood glucose in a euglycaemic range. If an individual is relatively insulin resistant, smaller amounts of glucose will need to be infused for a given amount of insulin to achieve euglycaemia.
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Advantages: The principal advantage of the glucose clamp in humans is that it directly measures whole body glucose disposal at a given level of insulinaemia under steady-state conditions. The approach is straightforward and there are a limited number of assumptions which are clearly defined. In research settings where assessing insulin resistance is of primary interest and feasibility is not an issue (e.g., study population < 100) it is appropriate to use the reference standard glucose clamp technique.
Limitations: The clamp test is labour intensive, technically difficult to perform, and expensive.
Another limitation of the HEC is that insulin sensitivity is measured only under a steady-state condition, and therefore, the test does not realistically portray dynamic conditions such as those occurring after normal meals.
2.5.2 FREQUENTLY SAMPLED INTRAVENOUS GLUCOSE TOLERANCE TEST (FSIGT) MINIMAL MODEL ANALYSIS.
The FSIGT was developed by Richard Bergman in 1979 and is based on minimal model of glucose and insulin kinetics. The model is applied to observed glucose and insulin values obtained from the FSIGT to obtain an insulin sensitivity index (SI). The SI value of the minimal model represents the fractional disappearance of glucose per insulin concentration unit over time.59
Advantages: It is not as labour intensive or as expensive to perform as the HEC and IR results from the FSIGT have good concordance rates with the HEC.59
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Limitation: The FSIGT is not as technically simple or inexpensive as several of the oral or fasting specimen-only methods, which somewhat limits it’s utility for large epidemiologic studies.
2.5.3 INSULIN SUPPRESSION TEST (IST)
Procedure: The insulin-suppression test was introduced by Shenet et al.40 in 1970. After an overnight fast, somatostatin or the somatostatin analogue octreotide is intravenously infused to suppress endogenous secretion of insulin and glucagon. Simultaneously, insulin (25 mU/m2/min) and glucose (240 mg/m2/min) are infused into the same antecubital vein over three hours. From the contralateral arm, blood samples for glucose and insulin determinations are taken every 30 min for 2.5 hours and then at 10 minutes intervals from 150 - 180 minutes of the IST. The constant infusions of insulin and glucose determine steady-state plasma insulin and glucose concentrations. The steady-state period is assumed to be from 150 - 180 min after initiation of the IST. The steady-state plasma glucose concentration will be higher in insulin resistant subjects and lower in insulin sensitive subjects. The IST provides a direct measure of the ability of exogenous insulin to mediate disposal of an intravenous glucose load under steady-state conditions where endogenous insulin secretion is suppressed.
Advantages: The steady-state plasma glucose is a highly reproducible direct measure of metabolic actions of insulin that is less labour-intensive and less technically demanding than the glucose clamp. Estimates of insulin sensitivity determined by steady-state plasma glucose correlate well with reference standard glucose clamp estimates in normal subjects (r = 0.93) and in patients with type 2 diabetes mellitus (r = 0.91).60 In research settings where assessing insulin sensitivity/resistance is of primary interest and feasibility is not an issue, it is appropriate to use
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the IST. Moreover, the IST can be used for larger populations that may pose difficulties for application of the glucose clamp 60
Limitations: The limitations of the IST are similar to those described above for the glucose clamp (with the exception that the IST is less technically demanding). Steady-state plasma glucose under ideal conditions determines primarily skeletal muscle insulin sensitivity and is not designed to reflect hepatic insulin sensitivity.
2.5.4 HOMEOSTASIS MODEL ASSESSMENT (HOMA)
The original homeostasis model assessment (HOMA-IR) is a relatively simple mathematical index for assessing insulin resistance, which is calculated by using fasting insulin and glucose values. HOMA β cell is a marker of basal insulin secretion of pancreatic β-cells.
HOMA-IR = (FPG × FPI)/22.5
HOMA-β (%) = (20 × FPI)/ (FPG – 3.5)
Where FPG is the fasting plasma glucose value (measured in mmol/L), FPI is the fasting plasma insulin value (measured in mU/L), and 22.5 is a constant.4 The HOMA has a range of approximately 2 to 15 with higher scores indicating increasing insulin resistance.4
Advantages: The major advantage of HOMA models is that it requires only one blood draw from a fasting patient. It does not require extensive technical expertise and constitute a much lower cost per subject when compared with the HEC, making the HOMA model much more practical for use in large-scale epidemiologic studies and for clinical situations.
Limitations: The major disadvantage is that the method fails to provide information about the stimulated glucose and insulin systems. It provides information only about what is occurring with homeostatic mechanisms in the fasting state, largely reflecting insulin’s effect on hepatic
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glucose production. The method does not address the peripheral action of insulin when stimulated with a glucose challenge.
C-peptide (CP) measurement has been used as an alternative to insulin for assessing pancreatic insulin secretion in HOMA. Both insulin and CP originate from the pancreatic cells, and both are secreted in equal amounts from the beta cell; however, CP is only minimally extracted or metabolized by the liver.5 The model for insulin resistance is the HOMA-IR (CP) while that for islet cell function is the HOMA-islet (CP) and they are both calculated as shown.5
HOMA-IR (CP) = (1.5 + FPG) x Fasting serum CP 2800
HOMA-islet (CP) in non-DM subjects = 0.27 x Fasting serum CP (FPG - 3.5) + 50
HOMA-islet (CP) in DM subjects = 0.27 x Fasting serum CP (FPG - 3.5)
Fasting plasma glucose and serum CP in the above formulae for the modified HOMA indices are in mmol/L and pmol/L respectively. The modified HOMA-IR (CP) and HOMA-islet (CP) has been found to correlate strongly with HOMA-IR and HOMA-islet respectively.5
2.5.5 QUANTITATIVE INSULIN SENSITIVITY CHECKS INDEX (QUICKI) MODEL.
The QUICKI was first proposed by Katz et al as a relatively easy mathematical index for assessing insulin sensitivity.
QUICKI is defined by the following formula: 1/log FPI + log FPG,
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Where FPI= fasting plasma insulin value (in μU/ml) and FPG = fasting plasma glucose value (in mg/dl).
Katz and colleagues found that, in the study population in which they developed the QUICKI, the mean QUICKI value for non-obese subjects was 0.382 ± 0.007, for obese subjects was 0.331
± 0.010, and for diabetic subjects was 0.304 ± 0.007.6 The advantages and limitations are similar to that of HOMA –IR.
2.5.6 FASTING GLUCOSE TO INSULIN RATIO
Fasting glucose to insulin ratio (GIR) has been considered as another useful method for assessing insulin resistance with high sensitivity and specificity particularly in patients with polycystic ovarian syndrome.12 This index however does not take into account the inappropriately low insulin secretion in the face of hyperglycaemia seen in diabetic subjects or glucose-intolerant subjects. Consequently, using fasting glucose to insulin ratio as a measurement of insulin resistance in patients with glucose intolerance or T2DM who have diminished pancreatic reserve may lead to erroneous results.
2.5.7 FASTING INSULIN AND C- PEPTIDE
Fasting insulin levels are often used as a clinical marker for insulin resistance. The AHA/
NHLBI statement noted that the European Group for the Study of Insulin Resistance uses the criterion of insulin levels greater than the 75th percentile of the population as indicative of insulin resistance.12,25 When measuring insulin levels, laboratory tests cannot distinguish between endogenous and exogenously administered insulin. C-peptide or proinsulin assays are
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more often used as valid markers of endogenous insulin secretion for those individuals taking insulin.
2.5.8 USE OF COMBINATION MARKERS TO ESTIMATE INSULIN RESISTANCE.
A combination of anthropometric, biochemical and imaging measures had been used in previous studies to estimate insulin resistance. 61,62 In a study carried out in Australia, a combination of fasting insulin, serum triglycerides and WHR were used to estimate IR and the combined method was found to have a better correlation with the clamp-derived IR (R2 0.58 versus 0.32 HOMA-IR) compared to HOMA-IR.61 Another study carried out in India used a simple scoring system called Karnataka Institute of Diabetology (KID) score to estimate IR among T2DM subjects. The KID scoring system was computed from a combination of BMI, WC, BP, triglycerides, FPG and fasting insulin. The scoring system was noted to have significant correlation with HOMA and QUICKI indices.62
40 Table 1: Methods of Measuring Insulin Resistance
Method Comments Advantages Limitations
Hyperinsulinaemic euglycaemic glucose clamp
Gold standard method for quantifying IR
Direct measure of insulin under steady-state conditions
Laborious, involves intra venous infusion of insulin, frequent blood sampling Homeostasis model
assessment
Assesses inherent beta cell function and IR
Simple, minimally invasive, predicts fasting steady-state G and I levels
Insulin resistance in subjects treated with Insulin needs further validation
QUICKI Mathematical
transformation of FPG and insulin
Consistent, precise index of
IR, minimally invasive
Normal range to be established for each laboratory due to significant inter laboratory
variations in insulin assay
FSIGT Indirect measure of IR Analysis using the computer
program
Multiple blood sampling Fasting insulin Most practical method
to measure IR
Detects IR before clinical disease appears
Lack of standardization of the insulin assay Procedure
GIR Comparable to IR
measured by the FSIGT
Highly sensitive and specific for
IR
Does not aptly reveal the physiology of IR G, Glucose; I, Insulin; IR, Insulin resistance; FPG, Fasting plasma glucose; FSIGT, Frequently Sampled Intravenous Glucose Tolerance Test (FSIGT) Minimal Model Analysis; GIR, Glucose insulin ratio;
QUICKI, Quantitative insulin sensitivity check index.
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