The author investigated the effect of some variables such as age, sex and the experience of past vaccination on the validity of PHA. The changing pattern of the validity with the change of PHA diagnostic criteria, and the relationship between PHA test result and RIA Ratio Unit were also studied. The results obtained were as follow; 1) No statistically significant difference was found in sensitivity, specificity and negative predictability by sex, but positive predictability was significantly higher in male than that in female. 2) Positive predictability was shown to become higher with the increase of age and nagative predictability was found to be significally different among age groups, but no statistically significant difference was found in sensitivity and specificity by age group. 3) Significantly low specificity and high positive predictability were found in past vaccined group, but no statistically significant difference was found in sensitivity and negative predictability between past vaccined group and non-vaccined group. 4) False negative cases by PHA were found to be the weak positive reactors by RIA and false positive rate of PHA was as high as 46.3 per cent. 5) Sensitivity and specificity of PHA at the diagnostic criteria of HBsAb titer 1:2 were 98.4% and 53.8% respectively, but after increasing the HBsAb titer to 1:64 as the diagnostic criteria the results were 60.0% and 95.6% respectively.
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