Calibration standards commercial assay kits

PCBs in soils and wastewaters can be rapidly screened on site or in the laboratory by immunoassay technique (Chapter 1.13). Immunoassay test kits are now commercially available from many suppliers. The samples can be tested at the calibration levels of 1 to 50 ppm. The kit primarily contains antibody-coated test tubes or magnetic particles, assay diluent, PCB-enzyme conjugate, a color-forming substance, and a solution to quench the reaction. The method does not distinguish accurately one Aroclor from another. PCBs can be measured semiquantitatively by comparing the optical density of the color formed in the sample against a set of calibration standards using a spectrophotometer. 

Commercial kits are usually supplied with calibration standard material. This may be in the form of a single vial of product from which a calibration curve can be constructed by serial dilution, or it may be a series of vials of ready-to-use standards with concentrations representative of the method s analytical range. For the purposes of assays used in drug development, it is often the case that the user will require a larger amount of calibration material, or a larger number of calibrator levels than is supplied. In the latter case, it is often possible to produce additional calibration standards from those provided either by dilution or by the mixing of those supplied. 

GH assays now use mouse monoclonal antibodies, and some of these assays are able to discriminate GH variants. Most procedures use recombinant-derived GH for labeling with a tracer and calibration material. The latter is usually prepared gravimetricaUy and verified by comparison with an international reference preparation (IRP), such as the World Health Organization s (WHO s) international standard, IRP 80/505 human growth hormone recombinant (hGHr), which has a potency of 3.3IU/mg of r-hGH, or other standard preparations, such as WHO IRP 66/217 or 88/624. The assay diluent, however, can vary considerably from one assay to another and is a potential source of bias some commercial kits use phosphate-buffered saline, and others use horse, calf, or human serum. 

At a minimum, documentation of the characterization and stability of a standard, such as a certificate of analysis (Co A) and/or a certificate of stability (CoS), is typically available from the suppliers. The certificate should be obtained and recorded. The quantity of reference standard is typically limited in commercial kits designed for research use, and it is not uncommon that the reference material values may differ substantially between lots and manufacturers [16]. Novel biomarkers rarely have established gold standards against which their potency and abundance can be calibrated. A comparison of available sources can be useful, and when validating an assay for advanced applications it is desirable to plan ahead to obtain and reserve a sufficient supply of the same reference material. The example in Fig. 6.5 compares three reference standard curves, each prepared from a concentrated stock solution from a commercial supplier, an in-house reference standard, and a commercial kit, respectively. The instrument responses (optical density, OD) were highest with the standard from the commercial stock, the lowest with the kit, while the in-house reference standard response was intermediate. In this case, either the same commercial stock or the in-house reference standard can be used throughout the clinical study. 

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