The strong ion difference
(SID) (apparent [SIDa] and effective [SIDe]) and strong ion gap
(SIG) provide a comprehensive method of evaluating acid-base status
in critically ill patients. The SID is the difference between strong cations and strong anions in plasma, while the SIG demonstrates the presence of unmeasured ions. This approach accounts for changes in a patient’s protein status, which is particularly important in those with burn
injuries. We hypothesized that the SIDa, SIDe, and SIG during the first 72 hours after admission would be predictive of mortality in burn
This study is a retrospective review of adults with 20% or greater total body surface area burns admitted during a 7-year period to a regional burn
center. SIDa, SIDe, and SIG were calculated at admission and for the first 3 days. These results were then compared with Acute Physiology and Chronic Health Evaluation II (APACHE II) and sepsis-related organ failure assessment (SOFA) scores.
A total of 113 patients met the criteria and had full data sets, with mean ± SEM age of 45.4 ± 1.4 years and total body surface area burn
of 41.4% ± 1.6%. Mortality was 27.4%. At admission, APACHE II remained most predictive of mortality (p
= 0.006). However, admission SIG (SIDa − SIDe) was also predictive of mortality on multivariate analysis (odds ratio, 1.11). Day 1 SIDa (Na+
) and SIDe ([1,000 × 2.46 × 10−11
] + [[albumin] × (0.123 × pH − 0.631)] + [[PO4
] × (0.309) × pH − 0.469)]) were also associated with mortality (odds ratio, 1.16 and 1.13 respectively), and SIDe with length of stay and ventilator days (p
The SID and SIG are predictive of mortality, hospital length of stay, and ventilator days in adult burn
patients. They also elucidate complex acid-base disorders.
LEVEL OF EVIDENCE
Prognostic study, level II.