Relevance of kinetic concepts to interpret a plasma concentration in clinical biochemistry

In clinical biochemistry a plasma concentration is measured to get informat ion either on the analyte input process or on the efficiency of a clearing organ. The interpretation of plasma concentration requires knowing (or assu ming) its physiological determinants i.e. : the analyte input rate into the plasma, the analyte output rate from the plasma and the size of the analyt e distribution space. In this review, the principles of interpretation of a spot plasma concentra tion are given taking into account kinetic concepts such as clearance, half -life of elimination, volume of distribution, binding to proteins... It is shown that clearance variation can be a confounding factor when asses sing an input rate (e.g.: enzyme efflux from an injured organ). Conversely, variation of the analyte production rate can be the confounding factor whe n indirectly assessing a clearance function from a spot plasma concentratio n (e.g. creatinine for kidney). Plasma analyte binding and interpretation o f a measured total plasma concentration can be very misleading if the relev ant concentration is only the free analyte concentration. Indeed the free p lasma concentration (the independent variable) can remain constant whereas the total plasma concentration (the dependent variable) can vary due to alt erations of the binding determinant. The meaning of half-lire is qualified and is shown to be a hybrid parameter which can be influenced both by the analyte clearance and distribution. It s use to assess a clearing efficiency can be misleading if the distribution space of the analyte has been modified.