For calculation of the sodium deficit in patients with hypovolaemic hyponatraemia the following formula is usually recommended: sodium deficit = 0.6 * BW * (140 – SNa + ) where BW is the body weight before dehydration and S Na + is the recorded serum sodium concentration. This formula does not take into account a d ecrease of body weight caused by dehydration (DBW). Based on follow-up of three patients with hypovolaemic hyponatraemia and theoretical analysis the following formula was derived: sodium deficit = 0.6 * BW * (140 – SNa + ) + DBW * SNa + . The values of sodium deficit calculated on the basis of this formula are significantly higher than those calculated on the basis of the commonly used formula.

        Key words: sodium deficit, hypovolaemic hyponatraemia.