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| CANTERBURY HEALTH LABORATORIES |
Steve Soule, Endocrinologist.
There has been an increase of around 50% in the number of requests received by Endolab for the measurement of plasma insulin in 2003-2004 compared with 2002-2003, with an annual cost approximating NZ$22,000. The measurement of insulin at the time of unexplained symptomatic hypoglycaemia is a crucial investigation which should never be omitted.
However the vast majority of requests are to investigate possible 'insulin resistance' or 'the metabolic syndrome', particularly in the context of polycystic ovary syndrome (PCOS). The term insulin resistance syndrome or metabolic syndrome refers to a cluster of vascular risk factors and is defined below, with three of the five abnormalities defining the syndrome:
| Risk factor | Cut-off level |
| Abdominal obesity | Women > 88cm Men >102cm |
| Fasting triglycerides | > 1.7 mmol/L |
| HDL | Women < 1.3 mmol/L Men < 1.0 mmol/L |
| Blood pressure | ≥ 130/85 mmHg |
| Fasting and 2hr glucose from 75g GTT | Fasting 6.1-7 and/or 2h 7.8-11.1mmol/L |
Thus measurement of plasma insulin is not helpful for the diagnosis of the syndrome. The reason is that there is currently no validated clinical test for detecting insulin resistance in the general population. The gold standard method (hyperinsulinaemic euglycaemic clamp) is a research procedure and although calculated indices based on fasting glucose and insulin (HOMA, QUICKI) correlate reasonably with the gold standard there are multiple flaws which limit their widespread clinical use, including changes in β-cell function with the development of diabetes (which alters the sensitivity of the test), normal physiological fluctuation in insulin levels and the lack of a standardised universal insulin assay. Perhaps most importantly, there are no data available which indicate that treating insulin resistance per se, however one may choose to define it, has any beneficial effects on patient outcome. Conversely, a wealth of data attests to the benefits of a variety of therapeutic measures in patients with the metabolic syndrome.
Hence, until further data emerges we would advise against routine measurement of either random or fasting plasma insulin in patients with possible metabolic or insulin resistance syndrome. Instead we emphasise the value of evaluating and treating the individual components of the metabolic syndrome (fasting lipids, BP, waist measurement and fasting ± 2hr glucose).
References