The homeostasis model assessment-estimated insulin resistance (HOMA-IR), developed by Matthews et al.  has been widely used for the estimation of insulin resistance in research. Compared with the “gold” standard euglycemic clamp method for quantifying insulin resistance , quantification using HOMA-IR is more convenient. It is calculated multiplying fasting plasma insulin (FPI) by fasting plasma glucose (FPG), then dividing by the constant 22.5, i.e. HOMA-IR = (FPI×FPG)/22.5 .
HOMA-IR tells us how hard the body is working to keep blood glucose from getting dangerously high. In other words, it tells us how much insulin pancreas has to produce in order to maintain blood glucose at a certain level.
Pre-diabetes can exist for a long time in your body without triggering the most common outward signs of diabetes (continual thirst, frequent urination, blurred vision, etc). And standard methods of detecting insulin resistance or pre-diabetes using glucose tolerance tests or an HbA1C percentage often show false negatives.
Here are some examples:
Fasting glucose: 90 mg/dL
Fasting insulin: 4 μIU/mL
HOMA-IR: (90 x 4) / 405 = 0.88
Fasting glucose: 82 mg/dL
Fasting insulin: 14 μIU/mL
HOMA-IR: (82 x 14) / 405 = 2.83
A1c = (46.7 + average_blood_glucose) / 28.7
Patient A’s fasting glucose is higher than Patient B’s, but Patient A’s insulin is much lower. By taking both glucose and insulin into account, the HOMA-IR scores show us that even with a lower fasting glucose, Patient B is at greater risk for metabolic complications down the road.
Because of the wide variation in insulin levels during a GTT, most endocrinologists evaluate the fasting insulin level only.