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Correction of the anion gap
for albumin in order to detect occult tissue anions in shock.
Hatherill M, Waggie Z,
Purves L, Reynolds L, Argent A.
Arch Dis Child 2002 Dec;87(6):526-9.
Paediatric Intensive Care Unit, School
of Child & Adolescent Health, University of Cape Town, and Red Cross War
Memorial Childrenrsquo, South Africa.
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BACKGROUND:
It is believed that hypoalbuminaemia confounds interpretation of the anion gap
(AG) unless corrected for serum albumin in critically ill children with shock.
Aim: To compare the ability of the AG and the albumin corrected anion gap (CAG)
to detect the presence of occult tissue anions. METHODS: Prospective
observational study in children with shock in a 22 bed multidisciplinary
paediatric intensive care unit of a university childrenrsquo;s hospital. Blood
was sampled at admission and at 24 hours, for acid-base parameters, serum
albumin, and electrolytes. Occult tissue anions (lactate + truly "unmeasured"
anions) were calculated from the strong ion gap. The anion gap ((Na + K) - (Cl +
bicarbonate)) was corrected for serum albumin using the equation of Figge: AG +
(0.25 x (44 - albumin)). Occult tissue anions (TA) predicted by the anion gap
were calculated by (anion gap - 15 mEq/l). Optimal cut off values of anion gap
were compared by means of receiver operating characteristic (ROC) curves. Ninety
three sets of data from 55 children (median age 7 months, median weight 4.9 kg)
were analysed. Data are expressed as mean (SD), and mean bias (limits of
agreement). RESULTS: The incidence of hypoalbuminaemia was 76% (n = 42/55). Mean
serum albumin was 25 g/l (SD 8). Mean AG was 15.0 mEq/l (SD 6.1), compared to
the CAG of 19.9 mEq/l (SD 6.6). Mean TA was 10.2 mmol/l (SD 6.3). The AG
underestimated TA with mean bias 10.2 mmol/l (4.1-16.1), compared to the CAG,
mean bias 5.3 mmol/l (0.4-10.2). A clinically significant increase of TA >5
mmol/l was present in 83% (n = 77/93) of samples, of which the AG detected 48%
(n = 36/77), and the CAG 87% (n = 67/77). Post hoc ROC analysis revealed optimal
cut off values for detection of TA >5 mmol/l to be AG >10 mEq/l, and CAG
>15.5 mEq/l. CONCLUSION: Hypoalbuminaemia is common in critically ill
children with shock, and is associated with a low observed anion gap that may
fail to detect clinically significant amounts of lactate and other occult tissue
anions. We suggest that the albumin corrected anion gap should be calculated to
screen for occult tissue anions in these children.
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