Validity of whole blood osmolality measurement in sick neonates

A comparison of heparinized whole blood with plasma osmolality measurements was performed on 100 sick newborns using a vapor pressure osometer. Aliquots of blood samples which had been collected into 1-ml syringes containing sodium heparin for blood gas analyses were used for osmolality determinations. There was excellent correlation between the two methods (r = 0.993). A total of 85 samples agreed within 2.0 units. In 38 samples the results obtained by the two methods were identical. The maximum observed discrepancy, present in only 2 samples, was 4 mosmol. Thus, the measurement of whole blood or plasma osmolality provides comparable information. The simplicity, the small volume of the blood sample required and the rapidity of the method make the determination of whole blood osmolality particularly useful in newborns requiring intensive care.