J. Levraut et al., RELIABILITY OF ANION GAP AS AN INDICATOR OF BLOOD LACTATE IN CRITICALLY-ILL PATIENTS, Intensive care medicine, 23(4), 1997, pp. 417-422
Objective: To evaluate the sensitivity, specificity, and predictive va
lues of an elevated anion gap as an indicator of hyperlactatemia and t
o assess the contribution of blood lactate to the serum anion gap in c
ritically ill patients. Design: Prospective study. Setting: General in
tensive care unit of a university hospital. Patients: 498 patients, no
ne with ketonuria, severe renal failure or aspirin, glycol, or methano
l intoxication. Measurements and results: The anion gap was calculated
as [Na+] - [Cl-] - [TCO2]. Hyperlactatemia was defined as a blood lac
tate concentration above 2.5 mmol/l. The mean blood lactate concentrat
ion was 3.7 +/- 3.2 mmol/l and the mean serum anion gap was 14.3 +/- 4
.2 mEq/l. The sensitivity of an elevated anion gap to reveal hyperlact
atemia was only 44 % [95 % confidence interval (CI) 38 to 50], whereas
specificity was 91 % (CI 87 to 94) and the positive predictive value
was 86 % (CI 79 to 90). As expected, the poor sensitivity of the anion
gap increased with the lactate threshold value, whereas the specifici
ty decreased [for a blood lactate cut-off of 5 mmol/l: sensitivity 67
% (CI 58 to 75) and specificity = 83 % (CI 79 to 87)]. The correlation
between the serum anion gap and blood lactate was broad (r(2) = 0.41,
p < 0.001) and the slope of this relationship (0.48 +/- 0.026) was le
ss than 1 (p < 0.001). The serum chloride concentration in patients wi
th a normal anion gap (99.1 +/- 6.9 mmol/l) was comparable to that in
patients with an elevated anion gap (98.8 +/- 7.1 mmol/l).Conclusions:
An elevated anion gap is not a sensitive indicator of moderate hyperl
actatemia, but it is quite specific, provided the other main causes of
the elevated anion gap have been eliminated. Changes in blood lactate
only account for about half of the changes in anion gap, and serum ch
loride does not seem to be an important factor in the determination of
the serum anion gap.