Description

Dear Sir,

In Diagnosis of Primary Antibody Deficiency, CPD Bulletin Immunology and Allergy 1999; 1(1): 6-9, anti-pneumococcal antibody levels were expressed in u/ml in Table 3b. I was previously unaware of the use of units/ml in this situation.

Standardisation of an assay of polyclonal antibodies against pneumococcal carbohydrate antigens of many different serotypes is difficult, the results of some ELISA assays having previously been expressed as the ratio of the optical density of the test serum (minus that of a negative control serum) to the optical density of a positive normal control serum (corrected by the same negative control serum), and expressed as % of control serum. The method used in Birmingham was modified by Windebank et al, who used a high positive control serum, and in an identical method of calculation used the term Specific binding index rather than simply % of control. Do the units/ml refer to a recently defined standard of which Immunologists should be aware, or another new name for % of a control preparation?

Sincerely

Reference:

1. Windebank KP, Faux JA, Chapel HM. ELISA determination of IgG antibodies to pneumococcal capsular polysaccharides in a group of children. Journal of Immunological Methods 1987; 104: 143-148.

Reply from the Editor

NEQAS in Sheffield made a serum available about a year ago which they assigned to contain 70 units/mL of IgG antibody to the pooled Pneumovax antigen. This pooled antigen containing 23 serotypes of pneumococcal polysaccharides has been used for some years in the Binding Site ELISA assay. This is equivalent to about 70mg/L of antibody in our own laboratory at the Royal Free, London. It would seem sensible if we all moved towards using units/mL so that we can base our assays on the same standard, which should assist us in our efforts to assess the usefulness of measuring these antibodies in different centres.