There is no normal value for free psa and it is usually necessary to test both free psa and total psa, as a routine, at the same time. When the serum total psa value is between 4-10ng/mL, the level of free psa is to be referred to, when the level of free psa is negatively correlated with the incidence of prostate cancer. In general, a ratio of free psa to total psa greater than 0.16 is recommended as a normal reference value in China. In more detail, when the ratio of free psa to total psa is less than 0.1, the possibility of prostate cancer to occur should be more than 50%. When the ratio of free psa to total psa is greater than 0.25, the likelihood of prostate cancer occurring is less than 10%.