Table 1.
Differences in studied population, measured ions, calculation of variables, and references among articles comparing the two approaches.
| Author | Studied population | n | Measurements of electrolytes | What is AGc corrected for? | Reference of AGc (mmol/L) | Calculation of SIG | Reference of SIG | Main results and comments |
|---|---|---|---|---|---|---|---|---|
| Fencl et al.40 | ICU | 152 | Blood gas analyzer | Alb | >21 (based on healthy subjects | Mg2+, Ca2+, Alb–, Pi– | >14 (based on healthy subjects) | While unmeasured strong anions represented by SIG detected 35% of patients with normal BE, AGc found 59% of hidden metabolic acid–base disturbances |
| Cusack et al.41 | ICU | 100 | Blood gas analyzer | Alb | >12 | Mg2+, Ca2+, Alb–, Pi– | >0 | SIG and SIDe in Stewart principle appear to offer no advantage in prediction of outcome |
| Rocktaeschel et al.42 | ICU | 300 | Central laboratory | Alb | N/A | Mg2+, Ca2+, Lac–, Alb–, Pi– | N/A | AUROC curves of AGc, SIDe, and SIG for mortality prediction were relatively small |
| Hucker et al.43 | Accident and emergency department | 1424 | Central laboratory | Alb | N/A | Mg2+, Ca2+, Lac–, Alb–, Pi– | N/A | All of each single variable in both approach have similar and unreliable predictive value |
| Martin et al.24 | Surgical ICU, trauma | 2152 | Central laboratory | Alb, Lac | N/A | Mg2+, Ca2+, Lac–, Alb–, Pi– | >0 | AUROC for mortality was strong for AGc with AUROC values of 0.68 compared with that for SIG (0.54) |
| Gunnerson et al.27 | ICU | 9799 | Central laboratory | Alb, Pi, Lac | N/A | Mg2+, Ca2+, Lac–, Alb–, Pi– | >50% of SBE | AGc identified only 84% of patients classified as SIG acidosis SIG, not AGc, was an independent predictors of mortality (OR 1.065; 95% CI 1.03–1.10; p = 0.001) |
| Dubin et al.33 | ICU | 935 | Central laboratory | Alb | 3 SD above or below the mean of 7 normal volunteers | Mg2+, Ca2+, Alb–, Pi– | 3 SD above or below the mean of 7 normal volunteers | When AGc was included in acid–base analysis, the Stewart approach did not offer any diagnostic or prognostic advantages |
| Kaplan and Kellum28 | ICU, major trauma | 78 | Central laboratory | Alb, Pi, Lac | N/A | Mg2+, Ca2+, Lac–, Alb–, Pi– | N/A | Although AGc had acceptable ROC curves (0.86) for 28-day mortality, it was significantly inferior to SIG (0.96) (p = 0.018) |
| Boniatti et al.29 | ICU (medical and surgical) | 175 | Central laboratory | Alb, Pi, Lac | >=17 | Mg2+, Ca2+, Lac–, Alb–, Pi– | >2 | There was significant difference between survivors and non-survivors in SIG (p = 0.01), but not in AGc (p = 0.11) |
| Abdulraof Menesi et al.44 | Kidney transplant | 83 | Central laboratory | Alb | >16 | Mg2+, Ca2+, Alb–, Pi– | >3 | A greater percentage of patients presented with an increase in unexplained anions by SIG than by AGc (42 vs 32%, respectively) (p value; N/A) |
| Ratanarat et al.45 | Medical and surgical ICU | 410 | Blood gas analyzer | Alb | >12 | Mg2+, Ca2+, Alb–, Pi– | >0 | According to ROC curves, the predictive ability to discriminate between survivors and non-survivors of AGc and SIG were 0.72 and 0.67, respectively |
| Zheng et al.6 | Nephrology ICU, metabolic acidosis | 78 | Central laboratory | Alb | N/A | Mg2+, Ca2+, Lac–, Alb–, Pi– | N/A | SIG value was associated with mortality at 24 h, 72 h, 1 week, 1 month, and 3 months after acute kidney injury, whereas AGc was not associated with mortality at each follow-up |
| Antonogiannaki et al.46 | Emergency department | 365 | Central laboratory | Alb | >17 (based on healthy volunteers) | Mg2+, Ca2+, Alb–, Pi– | >6 (based on healthy volunteers) | Significantly fewer patients with unmeasured anions acidosis were identified with AGc than those with SIGc (p=.0001) |
| Ho et al.47 | ICU | 6878 | Blood gas analyzer | Alb | N/A | Mg2+, Ca2+, Alb–, Pi– (, Lac–) | N/A | The abilities to predict hospital mortality in SIG (AUROC 0.52) and SIDe (0.63) are modest, whereas AGc (0.67) and BE (0.69) has stronger ability to differentiate between survivors and non-survivors |
| Morgan et al.48 | CPB | 60 | Blood gas analyzer | Alb | >20 | Ca2+, Lac–, Alb–, Pi– | >4 | AUROC of SIG for detecting “unmeasured anions” was significantly higher than that of AGc (0.81 vs 0.79; p = 0.048) |
| Guérin et al.49 | Chronic respiratory failure | 128 | Central laboratory | Alb | 8 healthy volunteers | Mg2+, Ca2+, Lac–, Alb–, Pi– | 8 healthy volunteers | The Stewart approach detected high SIDe in 13% of normal SBE and in 20% of normal AGc, and low SIDe in 22% of non-elevated HCO3−, providing better diagnostic performance |
| Shen et al.50 | Acute pancreatitis | 186 | Central laboratory | Alb, Lac | 13 health volunteers | Mg2+, Ca2+, Lac–, Alb–, Pi– | 13 health volunteers | SIG, but not AGc, had significant independent correlations with disease severity |
ICU: intensive care unit; AGc: corrected anion gap; BE: base excess; SBE: standard base excess; SID: strong ion difference; SIDe: effective strong ion difference; SIG: strong ion gap; SIGc: corrected strong ion gap; Mg: magnesium; Ca: calcium; Alb: albumin; Pi: inorganic phosphate; Lac: lactate; HCO3: bicarbonate; AUROC: area under receiver operating characteristic curve; ROC: receiver operating characteristic; N/A: not applicable; OR: odds ratio; CI: confidential interval; SD: standard deviation.