Blood and plasma volumes determined by carbon monoxide gas, 99mTc-labelled erythrocytes, 125I-albumin and the T 1824 technique.

The total amount of circulating haemoglobin was measured in 12 subjects using a direct carbon monoxide (CO)-technique. O2 plus 50 ml CO gas was rebreathed in a small closed system for 10 min. Carboxyhaemoglobin (HbCO)% was measured with a diode-array spectrophotometer before and after the rebreathing. delta HBCO% and the amount of CO in moles (nCO) were used to calculate the total amount of circulating haemoglobin. Blood volume was calculated by dividing this figure with the haemoglobin concentration and plasma volume by multiplying the blood volume with 1-haematocrit. The calculated blood and plasma volumes were compared with the simultaneously measured volumes by 99mTc-labelled erythrocytes, 125I-albumin and T 1824 (Evans Blue). Mean blood volume determined with CO was 4557 ml (3251-6576 ml) compared with 4527 ml (3390-6527 ml) with 99mTc-labelled erythrocytes (r = 0.97). Mean plasma volume by T 1824 was 2895 ml (1972-3658 ml) vs 2898 ml (1815-3714) ml using 125I-albumin, (r = 0.99). The plasma volumes calculated from the blood volumes determined by the erythrocyte-labelling methods were 5-10% lower than those measured with labelled albumin. There was a better correlation between the plasma volumes by the albumin methods and by the CO-technique (r = 0.98 and r = 0.97, respectively) than between the plasma volumes by the albumin methods and by 99mTc-erythrocytes (r = 0.90 and r = 0.87, respectively).