Ratio methods by comparison to a reference serum

Ratio methods by comparison to a reference serum Different ratio methods have been developed, such as the positive negative (P/N) ratio, the ratio of the areas under the dose-response curves of the test serum and of a reference serum, and the percent of positive method. 

The percent of positive method expresses the absorbance of the test-sample as a percentage of that of a positive reference serum, measured simultaneously. This method is, therefore, quantitative and quite simple, does not require a cut-off value and fluctuates less than the P/N ratio method. However, the choice of the positive reference serum is very important, since a reference serum with a very high activity (i.e. high absorbance) will decrease the percentage of apparent positive sera. Results are not linearly proportional to the titre. 

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