A method of determining an immunosuppressant drug in a sample suspected of containing the immunosuppressant drug, the method comprising a non-agglutination immunoassay comprising: (a) providing in combination in a medium: (i) the sample, (ii) a releasing agent for releasing the immunosuppressant drug from endogenous binding substances, wherein the releasing agent is selected from the group consisting of one or more of a structural analog of the immunosuppressant drug and a hemolytic agent, (iii) an antibody reagent comprising an antibody for the immunosuppressant drug, (iv) a pretreatment agent selected from the group consisting of hydroxyphenyl-substituted C1-C5 carboxylic acids and metallic salts thereof and halogen-substituted C1-C5 carboxylic acids and metallic salts thereof in an amount effective to enhance the accuracy of the non-agglutination immunoassay; (b) incubating the medium under conditions for binding of the antibody for the immunosuppressant drug to the immunosuppressant drug suspected of being in the sample, wherein the incubation temperature is 5°C to 99°C and the time period for the incubation is 0.2 seconds to 24 hours, (c) examining the medium for the presence of a complex comprising the immunosuppressant drug and the antibody for the immunosuppressant drug, the presence and/or amount of the complex indicating the presence and/or amount of the immunosuppressant drug in the sample, wherein the immunosuppressant drug is selected from the group consisting of cyclosporine, everolimus, sirolimus and tacrolimus. (a) providing in combination in a medium: (i) the sample, (ii) a releasing agent for releasing the immunosuppressant drug from endogenous binding substances, wherein the releasing agent is selected from the group consisting of one or more of a structural analog of the immunosuppressant drug and a hemolytic agent, (iii) an antibody reagent comprising an antibody for the immunosuppressant drug, (iv) a pretreatment agent selected from the group consisting of hydroxyphenyl-substituted C1-C5 carboxylic acids and metallic salts thereof and halogen-substituted C1-C5 carboxylic acids and metallic salts thereof in an amount effective to enhance the accuracy of the non-agglutination immunoassay; (i) the sample, (ii) a releasing agent for releasing the immunosuppressant drug from endogenous binding substances, wherein the releasing agent is selected from the group consisting of one or more of a structural analog of the immunosuppressant drug and a hemolytic agent, (iii) an antibody reagent comprising an antibody for the immunosuppressant drug, (iv) a pretreatment agent selected from the group consisting of hydroxyphenyl-substituted C1-C5 carboxylic acids and metallic salts thereof and halogen-substituted C1-C5 carboxylic acids and metallic salts thereof in an amount effective to enhance the accuracy of the non-agglutination immunoassay; (b) incubating the medium under conditions for binding of the antibody for the immunosuppressant drug to the immunosuppressant drug suspected of being in the sample, wherein the incubation temperature is 5°C to 99°C and the time period for the incubation is 0.2 seconds to 24 hours, (c) examining the medium for the presence of a complex comprising the immunosuppressant drug and the antibody for the immunosuppressant drug, the presence and/or amount of the complex indicating the presence and/or amount of the immunosuppressant drug in the sample, wherein the immunosuppressant drug is selected from the group consisting of cyclosporine, everolimus, sirolimus and tacrolimus.