A method for assessing a condition related to expression of a cell membrane protein of interest in a subject by quantitative measurement of the expression level of said CMP in the erythrocyte membranes of said subject, wherein the normal membrane expression range of said CMP in the erythrocyte membrane is pre-determined or known and wherein the condition in said subject is related to altered expression of said CMP in a tissue other than erythrocytes in said subject, said method comprising the steps of - providing a blood sample taken from said subject, said blood sample comprising erythrocytes, - preparing an erythrocyte test sample from the blood sample, wherein one or more epitope(s) of said CMP present in the erythrocyte membrane is/are made available to a CMP binding agent capable of specifically binding to said one or more epitope(s) and wherein the erythrocytes are whole cells including intact red blood cell fraction and ghost red blood cell fraction, - adding the CMP binding agent to the test sample under conditions wherein said CMP binding agent is specifically bound to said one or more epitope(s) of the CMP present in the erythrocyte membrane in a saturating amount, - obtaining signal elicited by the specific binding in a cellular detection experiment wherein the erythrocyte membranes are presented in the form of whole cells and wherein the CMP binding agent is bound to said CMP present in the erythrocyte membrane in the test sample, wherein either one or both of (a) a signal elicited by the binding events in intact red blood cell fraction and (b) a signal elicited by the binding events in ghost red blood cell fraction is/are obtained, wherein preferably the intact red blood cell fraction and the ghost red blood cell fraction are separated, - converting the obtained signal into a value correlating with the amount of the molecules of said CMP present in the membrane of said erythrocytes in each fraction, wherein said value is considered as the membrane expression level of said CMP, - comparing the obtained membrane expression level of said CMP in said subject with the normal membrane expression range of said CMP in the erythrocyte membrane, and a lack of difference from the normal membrane expression range is considered as indicative of a normal condition in said subject, a difference from the normal membrane expression range is considered as indicative of a condition different from normal in said subject. - providing a blood sample taken from said subject, said blood sample comprising erythrocytes, - preparing an erythrocyte test sample from the blood sample, wherein one or more epitope(s) of said CMP present in the erythrocyte membrane is/are made available to a CMP binding agent capable of specifically binding to said one or more epitope(s) and wherein the erythrocytes are whole cells including intact red blood cell fraction and ghost red blood cell fraction, - adding the CMP binding agent to the test sample under conditions wherein said CMP binding agent is specifically bound to said one or more epitope(s) of the CMP present in the erythrocyte membrane in a saturating amount, - obtaining signal elicited by the specific binding in a cellular detection experiment wherein the erythrocyte membranes are presented in the form of whole cells and wherein the CMP binding agent is bound to said CMP present in the erythrocyte membrane in the test sample, wherein - converting the obtained signal into a value correlating with the amount of the molecules of said CMP present in the membrane of said erythrocytes in each fraction, wherein said value is considered as the membrane expression level of said CMP, - comparing the obtained membrane expression level of said CMP in said subject with the normal membrane expression range of said CMP in the erythrocyte membrane, and