Relationship between digoxin concentrations in serum and saliva.

The concentration of digoxin in serum and saliva was determined in 18 patients receiving digoxin. Unlike serum, it was necessary to extract saliva with chloroform in order to quantitate digoxin levels accurately. An excellent linear correlation (r = +0.988, p less than 0.001) was observed between the saliva and serum digoxin concentrations. This indicates that saliva digoxin concentrations can be used to monitor digoxin therapy, particularly in patients in whom blood sampling is inconvenient or difficult. The saliva/serum ratio for digoxin concentration was 0.78 plus or minus 0.07 (SD). Since the digoxin binding to plasma proteins is 23%, it is the free drug that is in equilibrium between serum and saliva.