Determination of ammonia in ear-lobe capillary blood is an alternative to arterial blood ammonia

Abstract

Blood ammonia determination is a laboratory test to diagnose hepatic encephalopathy. Arterial blood is superior to peripheral venous blood ammonia because of ammonia metabolism in muscle. We have compared capillary with arterial whole blood ammonia as capillary sampling is an attractive alternative. Ear-lobe capillary blood ammonia (ECA) was determined in all 173 persons studied, fingertip capillary blood ammonia (FCA) in 46 of these and arterial blood ammonia (AA) in 113. Of the 173, 60 were healthy (H), 64 were patients, not liver diseased (NLD) and 49 had liver disease (LD). Reference values, median and ranges, μmol NH 3-N/1: AA, NLD, n = 64: 17 (7–42); ECA, H = NLD ( P = 0.9), n = 124: 20 (7–45); FCA, H = NLD ( P = 0.8), n = 33: 70 (29–151). Within the NLD group ( n = 64) AA values (range 7–42) were little but significantly lower than the ECA values (range 7–45, P = 0.002). FCA NLD > AA NLD ( n = 14, P < 0.0001); FCA H + NLD > ECA ( n = 33, P < 0.0001). AA correlated very well with ECA, r = 0.87 ( n = 113, P < 0.0001) and less well with FCA, r = 0.56 ( n = 27, P < 0.01). ECA correlated with FCA, r = 0.51 ( n = 46, P < 0.001). Ear-lobe capillary blood ammonia thus accurately reflects arterial ammonia and is an attractive alternative. The higher fingertip ammonia may be due to contamination with ammonia-rich sweat from finger grooves, regardless of the precautions taken.