pH lability in serum during equilibrium dialysis.

Changes in pH were determined in previously frozen normal human serum during dialysis against sodium phosphate, Krebs Ringer phosphate or Krebs Ringer bicarbonate buffers of pH 7.4. Serum was either untreated (native) or adjusted to pH 7.4 before dialysis. pH in native serum was 7.7-7.9 before dialysis, showed a decrease after 1 h, and an increase after 3 h. pH-adjusted serum showed a continuous pH increase during dialysis. The increase in serum pH during dialysis was larger at 37 degrees C than at 22 degrees C, larger at low than at high buffer molarity, and larger in native than in pH-adjusted serum. The observed changes in serum pH during dialysis are associated with unacceptably large errors in unbound fraction in serum for a number of important drugs. Drug binding determination in serum by equilibrium dialysis should be performed with buffers providing appropriate and stable pH level.