An apparatus for correcting an initial measurement of the concentration of a first water soluble species dissolved in a water-based component of a biological fluid that also includes a second water soluble species and a volume occupying component, the initial measurement having been taken using an original sample of the fluid that had been diluted by an amount of additional aqueous solution to form a diluted sample is disclosed; the concentration of the second water soluble species is measured in an undiluted sample of the biological fluid to obtain a direct concentration and is measured using a diluted sample of the fluid to obtain an indirect concentration, and the initial measurement of concentration of the first species is adjusted based on a combination of the direct and indirect concentration determination of the second species. The apparatus includes a first measurement device, which may be a colorimeter or amperometer, for measuring the diluted first species concentration; a second measurement device, which may be an indirect potentiometer, for measuring the diluted second species concentration; a third measurement device, which may be an ion selective electrode, for measuring the undiluted second species concentration; in particular embodiments, additional measurement devices, which may be a gas sensor and a spectrophotometer, for measuring the concentration of species which may complex with second species, and a computer connected to the various measurement devices.