Capillary plasma ammonia concentration in neonates receiving total parenteral nutrition. Comparison with arterial and venous concentrations

Plasma ammonia concentration in neonates has routinely been determined using arterial or venous blood. Expected plasma ammonia values in capillary blood obtained by heelstick have not been determined. We compared ammonia levels in 20 sets of plasma from simultaneously drawn arterial, venous, and capillary blood in a group of neonates receiving total parenteral nutrition. Mean ammonia concentrations in venous (107 +/- 44) and capillary blood (112 +/- 33) were 45% and 51% higher, respectively, than corresponding arterial (74 +/- 22) values (P less than .001). Ammonia levels in blood obtained by venipuncture (Ven), however, did not correlate consistently with arterial (Art) values (r = .43; Art = 51 + 0.21 Ven; P greater than .05). In contrast, ammonia levels in capillary blood (Cap) correlated well with arterial values (r = .86; Art = 10.3 + 0.6Cap; P less than .001). Ammonia levels in neonates may be reliably interpreted using the latter regression equation when blood for analysis is obtained by a properly performed heelstick, allowing the preservation of arteries and veins, and sparing the infant from repetitive needle punctures. Ammonia levels in blood obtained by venipuncture do not adequately correlate with arterial values and therefore may be therapeutically misleading.