Commercial assays available for insulin-like growth factor I and their use in diagnosing growth hormone deficiency

Radioimmunoassays of insulin-like growth factor I (IGF-I) are commonly used for screening adults and children for growth hormone (GH) deficiency or excess. There are, however, many problems with such assays. Attempts to resolve these problems have focused on methods of separating IGF-I from its binding proteins, and on reducing inter- and intra-assay variability. In particular, the collection of sufficient high-quality normative data is a major difficulty in many laboratories. Clinical evaluation of assays is also problematic. IGF-I levels vary with age after puberty, and this is complicated by the maintenance of IGF-binding protein 3 levels by IGF-II. Generally, studies have shown that IGF-I is sensitive and specific for the diagnosis of acromegaly, but screening for GH deficiency (GHD) is less precise. The most commonly used commercial assays are immunoradiometric (IRMA) sandwich assays, using antibodies specific to IGF-I. IRMA assays are quick and accurate, and the two-site antibody reactivity produces a high degree of specificity. Additional techniques such as acid-ethanol extraction or saturation with IGF-II can improve reliability. More recently, the introduction of chemiluminescence has provided enhanced speed and sensitivity. The clinical use of these assays has provided a wealth of information regarding the diagnosis of GHD, and it may be possible to reduce the number of patients who require provocative GH testing. IGF-I assays are also of great use in monitoring GH replacement therapy. Despite the problems, IGF-I measurement is currently the best indirect method available for screening and monitoring patients with GHD.