A list of new or replacement reference materials is given in Table 1. Details of selected reference materials are given below to illustrate the range of issues considered by the Committee.

Yellow fever vaccine

Potency determination of yellow fever vaccines has historically been based on mouse LD⁵⁰ assays although in vitro plaque assays have been available and in routine use for some years. The need to improve standardization of yellow fever potency determinations led to a collaborative study performed by thirteen laboratories in eight countries to assess the suitability of candidate preparations for an International Standard and the relationship between the two assay methods. On the basis of the results of the collaborative study, the Committee established a preparation, in ampoules coded 99/616, as the First International Standard for Yellow Fever Vaccine.

Data obtained in the study indicated that there was a consistent relationship between mouse and plaque assays. The Committee therefore supported a proposal to encourage manufacturers and control laboratories to include the standard in assays to evaluate its suitability for setting a minimum potency of 10^4.0 IU for yellow fever vaccines. Data should be collated by WHO and analysed to determine whether the potency specification given in the WHO Recommendations (4) should be amended.

Factor VIII, concentrate

Stocks of the current (recombinant) standard are likely to be exhausted within 12 months and there have been reports of difficulties in using recombinant material in assaying plasma-derived concentrates. For these reasons, and after extensive consultation, it has been decided that plasma-derived factor VIII will be the replacement material. A collaborative study among 38 laboratories in 21 countries to establish a common batch reference preparation was conducted and the Committee established the Seventh International Standard for Factor VIII, Plasma-derived, Concentrate, in ampoules coded 99/678, and an activity of 11.0 IU per ampoule.

Human interferon beta

The current International Standard for interferon beta is an impure preparation derived from human fibroblasts containing about 1% interferon. Other cytokines present influence the results of some assays. Accordingly, an extensive collaborative study of new and existing reference preparations for interferon beta has been performed by 16 laboratories in 8 countries. One candidate preparation, consisting of glycosylated interferon beta derived from Chinese hamster ovary (CHO) cells, gave a smaller inter-laboratory variability, compared with the current standard, with all but one sample examined. The Committee established the Third International Standard for Interferon Beta, Human, Recombinant, Glycosylated, in ampoules coded 00/572, and assigned a potency to it of 40 000 International Units per ampoule. Since stocks of the current standard remain, the Committee formally disestablished the Second International Standard for interferon beta, fibroblast, human, code number Gb23-902-531. The CHO-cell derived material is likely to continue to be available and is more suitable for calibration of future therapeutic products than the fibroblast material. However, it is not suitable for assay of the Ser-17 interferon beta analogue and the 1^st International Standard of interferon-beta Ser 17 mutein, Gxb02-901-535, will be retained.

Hepatitis B surface antigen

Because the current First International Standard for hepatitis B surface antigen is in need of replacement, a candidate and panel of dilutions were proposed. The aim of these reference materials is to aid regulatory authorities and manufacturers of HBsAg test kits in measuring analytical kit sensitivity by providing a standard with an internationally accepted unitage.

WHO collaborative studies were conducted and data demonstrated that the assigned values of the different HBsAg reference preparations differ considerably: 1 IU is equivalent to 0.58 PEI units (primary) or 0.43 PEI units (current) or 1.9 French ‘ng’ or 5.6 Abbott ‘ng’. However, it is noteworthy that in 1985, the relationship between IS and the primary PEI was nearly the same as was found in the current study: 1 IU = 0.55 PEI unit. This and related biochemical data indicate that there has been no drift in the IU over 18 years. Furthermore, the study showed that the prediluted panel provides a convenient resource for authorities to assess sensitivity, especially of rapid tests.

On the basis of the results obtained, the Committee established the candidate preparation as the Second International Standard for Hepatitis B surface antigen, in vials coded 00/588, with an assigned value of 33 IU per vial. The Standard contains antigen subtype adw2, genotype A. The Committee also established panel members A to D, in vials coded 01/400, 01/402, 01/404 and 01/ 406, which are 1 in 4, 1 in 16, 1 in 64 and 1 in 256 dilutions of the International Standard respectively, and panel member E, in vials coded 00/ 616, which consists of human re-calcified plasma, as a reference panel for hepatitis B surface antigen for use by national regulatory authorities in the assessment of the sensitivity of assay kits for the detection of the surface antigen.