Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1023 page)

   IGF-I and IGFBP-3 reference ranges are highly age dependent, and results must always be interpreted within the context of the patient’s age.

   Discrepant IGFBP-3 and IGF-I results can sometimes occur due to liver and kidney diseases; however, this is uncommon, and such results should alert laboratories and physicians to the possible occurrence of a preanalytic or analytic error.

   At this time, IGFBP-3 cannot be reliably used as a prognostic marker in breast, colon, prostate, or lung cancer.

   IGFBP-3 assays exhibit significant variability among platforms and manufacturers. Direct comparison of results obtained by different assays is problematic. Rebaselining of patients is preferred if assays are changed.

INSULIN-LIKE GROWTH FACTOR-I (IGF-I)

   Definition

   IGF-I is secreted by hypothalamus; release is mediated by growth hormone (GH) in many tissues, especially hepatocytes. It is a single polypeptide chain with 70-amino-acid residues with a molecular mass of 7,649 Da. It is structurally homologous to IGF-II and insulin. IGF-I circulates primarily in a high molecular weight tertiary complex with IGF-binding protein-3 (IGFBP-3) and acid-labile subunit. Plasma IGF-I levels are barely detectable at birth, rise gradually during childhood, peak during mid-puberty until approximately 40 years of age, and then decline gradually. Maternal plasma levels increase during pregnancy.

   
Normal range:
see Table 16.50; 0–7 days: <26 ng/mL; 8–15 days: <41 ng/mL.

TABLE 16–50. Normal Range of IGF-I

   Use

   Diagnosis of acromegaly and pituitary deficiency; preferable to GH because it is constant after eating and during the day

   Help determine optimum dosage of GH

   Screening other growth disorders

   Assessing nutritional status