Table 1.
Characteristics of the Tested IGF-I Assays as Provided by the Manufacturers
| Assay Name | Manufacturer | Automated | Tracer | International Standard Against Which the Assay Calibrated | Intra-assay CV | Inter-assay CV | LOQ or LOD (ng/ml) | Highest Measurable Value Without Dilution (ng/ml) | Reference Adult Population Recruited by the Manufacturer |
|---|---|---|---|---|---|---|---|---|---|
| iSYS | IDS | Yes | Acridinium ester | WHO/NIBSC 02/254 | 2.9% at 22 ng/ml | 5.4% at 22 ng/ml | 8.8 (LOQ) | 1200 | 6500 adults; reference values provided according to the method of Cole and Green (12) |
| 1.9% at 163 ng/ml | 3.9% at 163 ng/ml | ||||||||
| 4.2% at 304 ng/ml | 7.2% at 304 ng/ml | ||||||||
| LIAISON XL | DiaSorin | Yes | Isoluminol | WHO/NIBSC 02/254 | 5.1% at 70 ng/ml | 9.6% at 80 ng/ml | 3 (LOD) | 1500 | 1606 adults; reference values provided by age according to the method of Royston and Wright (14) |
| 3.5% at 183 ng/ml | 7.1% at 187 ng/ml | 10 (LOQ) | |||||||
| 3% at 589 ng/ml | 5.6% at 317 ng/ml | ||||||||
| IMMULITE 2000 | Siemens | Yes | Alkaline phosphatase | WHO/NIBSC | 3.9% at 77 ng/ml | 7.7% at 77 ng/ml | 20 (LOQ) | 1600 | 1499 pediatric and adult samples from an apparently healthy population (no indication is given concerning the respective numbers of adult and children) |
| First IRR 87/518 | 6.5% at 169 ng/ml | 5.4% at 169 ng/ml | |||||||
| 2.9% at 380 ng/ml | 7.4% at 380 ng/ml | ||||||||
| 3.0% at 689 ng/ml | 8.1% at 689 ng/ml | ||||||||
| 2.3% at 1053 ng/ml | 3.7% at 1053 ng/ml | ||||||||
| 2.4% at 1358 ng/ml | 4.7% at 1358 ng/ml | ||||||||
| IGFI-RIACT | Cisbio | No | 125I | WHO/NIBSC | 3.8% at 49 ng/ml | 3.8% at 39 ng/ml | 1 (LOD) | 900 | 693 adults 29–70 y |
| First IRR 87/518 | 3.4% at 162 ng/ml | 8.2% at 352 ng/ml | |||||||
| 3.2% at 496 ng/ml | 5.9% at 509 ng/ml | ||||||||
| Mediagnost | MEDIA | No | Peroxidase enzyme conjugate | WHO/NIBSC 02/254 | 5.7% at 138 ng/ml | 6.1% at 142 ng/ml | 1.9 (LOD) | 1050 | Based on the data reported by Blum and Breier (13) |
| ELISA | GNOST | 5.1% at 141 ng/ml | 6.8% at 174 ng/ml | ||||||
| 6.6% at 145 ng/ml | 2.2% at 494 ng/ml | ||||||||
| Mediagnost | MEDIA | No | 125I | WHO/NIBSC 02/254 | 4.6% at 56 ng/ml | 4.9% at 55 ng/ml | 2.6 (LOD) | 780 | Based on the data reported by Blum and Breier (13) |
| RIA | GNOST | 3.4% at 140 ng/ml | 6.2% at 140 ng/ml | The reference values for the different age ranges are the same as those used for the Mediagnost ELISA kit | |||||
| 2.5% at 180 ng/ml | 4.5% at 186 ng/ml |
Abbreviations: CV, coefficient of variation; LOD, limit of detection; LOQ, limit of quantification; NICSC, National Institute for Biological Standards and Control; WHO, World Health Organization.
These six assays are sandwich assays that use monoclonal antibodies directed against epitopes, whose exact nature is not disclosed by the manufacturers. In all cases, IGFBPs are said to be removed by displacement of endogenous IGF-I by an excess of IGF-II (or analog) as initially proposed by Blum and Breier (13). The LOQ is the lowest amount of IGF-I that can be accurately quantified with an allowable error <20%. The LOD is the IGF-I concentration corresponding to the 95th percentile value from a number of determinations of IGF-I concentration in free serum samples.