Differing Methodology and Equations Used in Quantitating Immunoglobulins by Radial Immunodiffusion—A Comparative Evaluation of Reported and Commercial Techniques

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Abstract
Differing methods of plotting have been used to describe the results of radial immunodiffusion (RID). Two, logarithm of antigen concentration (log c) vs. precipitin-ring diameter (d) (Fahey) and log c vs. area (A or d2) (Mancini), are linear while rings are enlarging. Another, c vs. A (or c vs. d2), becomes linear when enlargement ceases at equivalence (Mancini). Because of methodological inaccuracy, I could not experimentally determine whether log c = d or log c = d2 is appropriate; either one or both semilogarithmic graphs may be linear as precipitin circles enlarge. I could confirm the c = d2 linearity at equivalence. Because smaller circles cease growth first, intermediate readings produce plots that are partially curved and partially straight. Commercial RID plates produce linear and curved plots of log c vs. d and of c vs. d2. They often create too small a diameter range for great accuracy. I conclude that semilogarithmic plots are of questionable value. For greatest accuracy plates should be designed for measurement at equivalence. At this time, a straight-line graph of c = d2 will include a maximal diameter range.

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