Correcting the anion gap for hypoalbuminaemia does not improve detection of hyperlactataemia

Albumin Corrected Anion Gap and the Risk of in-Hospital Mortality in Patients with Acute Pancreatitis: A Retrospective Cohort Study

Purpose

To explore the prognostic value of albumin corrected anion gap (ACAG) within 24 hours of admission to the intensive care unit (ICU) for acute pancreatitis (AP).

Patients and Methods

This was a retrospective cohort study. Adult AP patients admitted to ICU from June 2016 to December 2019 were included in the study, who were divided into three groups according to initial serum ACAG within 24 hours upon ICU admission: ACAG ≤ 14.87 mmol/L, 14.87 < ACAG ≤ 19.03 mmol/L, and ACAG > 19.03 mmol/L. The primary study outcome indicator was in-hospital mortality. Age, sex, Glasgow Coma Scale score, and Acute Physiology and Chronic Health Evaluation II (APACHE II) score were matched through propensity score matching (PSM) method to balance the baseline between the survivors and non-survivors. Multivariate Cox regression was used to determine the relationship between ACAG and in-hospital mortality.

Results

A total of 344 patients (of them 81 non-survivors) were analyzed in this study. Patients with higher ACAG intended to present significantly higher in-hospital mortality, APACHE II score, creatine, lower albumin, and bicarbonate. Multivariate Cox regression analysis after matching demonstrated that white blood cell count, platelet count, and higher ACAG were independently associated with higher in-hospital mortality (ACAG ≤ 14.87 as a reference, 14.87 < ACAG ≤ 19.03 mmol/L with HR of 2.34 and 95% CI of 1.15-4.76, ACAG >19.03 with HR of 3.46 and 95% CI of 1.75-6.84).

Conclusion

Higher ACAG was independently associated with higher in-hospital mortality in patients with AP after matching the baseline between the survivors and non-survivors.

Acid-Base Disorders in the Critically Ill Patient

Acid-base disorders are common in the intensive care unit. By utilizing a systematic approach to their diagnosis, it is easy to identify both simple and mixed disturbances. These disorders are divided into four major categories: metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis. Metabolic acidosis is subdivided into anion gap and non-gap acidosis. Distinguishing between these is helpful in establishing the cause of the acidosis. Anion gap acidosis, caused by the accumulation of organic anions from sepsis, diabetes, alcohol use, and numerous drugs and toxins, is usually present on admission to the intensive care unit. Lactic acidosis from decreased delivery or utilization of oxygen is associated with increased mortality. This is likely secondary to the disease process, as opposed to the degree of acidemia. Treatment of an anion gap acidosis is aimed at the underlying disease or removal of the toxin. The use of therapy to normalize the pH is controversial. Non-gap acidoses result from disorders of renal tubular H + transport, decreased renal ammonia secretion, gastrointestinal and kidney losses of bicarbonate, dilution of serum bicarbonate from excessive intravenous fluid administration, or addition of hydrochloric acid. Metabolic alkalosis is the most common acid-base disorder found in patients who are critically ill, and most often occurs after admission to the intensive care unit. Its etiology is most often secondary to the aggressive therapeutic interventions used to treat shock, acidemia, volume overload, severe coagulopathy, respiratory failure, and AKI. Treatment consists of volume resuscitation and repletion of potassium deficits. Aggressive lowering of the pH is usually not necessary. Respiratory disorders are caused by either decreased or increased minute ventilation. The use of permissive hypercapnia to prevent barotrauma has become the standard of care. The use of bicarbonate to correct the acidemia is not recommended. In patients at the extreme, the use of extracorporeal therapies to remove CO 2 can be considered.

Acquired pyroglutamic acidaemia in a critically ill patient with chronic paracetamol use: A case report

Pyroglutamic acid is an endogenous organic acid and a metabolite in the γ-glutamyl cycle, involved in glutathione metabolism. Accumulation of pyroglutamic acid is a rare cause of high anion gap metabolic acidosis. There are multiple risk factors for pyroglutamic acid accumulation, such as chronic paracetamol use and sepsis. In this case report, we discuss how we came to this diagnosis, how it was subsequently managed and why it is an important consideration for critically ill patients with risk factors who are likely to end up in an intensive care setting. Pyroglutamic acid recognition and treatment could benefit patients in the critically ill population as pyroglutamic acid is a rare cause of high anion gap metabolic acidosis, which is likely under-recognised and easily treated. Inappropriate management of metabolic disorders can contribute to patient morbidity and mortality. Therefore, the recognition and appropriate management of pyroglutamic acidaemia could benefit patients with risk factors for its development in a critical care setting.

Anion gap physiology and faults of the correction formula

DISCLAIMER

In an effort to expedite the publication of articles, AJHP is posting manuscripts online as soon as possible after acceptance. Accepted manuscripts have been peer-reviewed and copyedited, but are posted online before technical formatting and author proofing. These manuscripts are not the final version of record and will be replaced with the final article (formatted per AJHP style and proofed by the authors) at a later time.

PURPOSE

The anion gap is a calculated fundamental laboratory parameter used to identify and monitor acid-base disturbances. A recently popularized correction formula transforms the resulting integer to compensate for hypoalbuminemia and improve diagnostic yield. Clinical pharmacists should be aware of the underlying biochemistry, interpretation, and limitations of this formula to discern drug- and disease-related etiologies.

SUMMARY

The anion gap is utilized in most care settings, ranging from outpatient monitoring to inpatient intensive care units. Supported by decades of experience, the original anion gap derives its value from its simplicity. Applying the anion gap in metabolic acidosis can help narrow differential diagnosis and detect concomitant acid-base disorders. To account for hypoalbuminemia and potential missed diagnoses, a correction formula was developed to improve sensitivity. Yet, the law of electroneutrality ensures that hypoalbuminemia is already accounted for in the original anion gap, and the proposed correction formula was derived from samples unrepresentative of human physiology. Evidence from clinical trials shows no benefit from applying the correction formula.

CONCLUSION

There is no advantage to correcting the anion gap, and such correction may increase the risk of misinterpretation or error. Clinicians should understand these limitations when diagnosing or trending acid-base disturbances.

Obesity, Anion Accumulation, and Anion Gap Metabolic Acidosis: A Cohort Study

Background

Obesity is associated with low serum bicarbonate, an indicator of metabolic acidosis and a CKD risk factor. To further characterize acid-base disturbance and subclinical metabolic acidosis in this population, we examined prospective associations of body mass index (BMI) with elevated anion gap and whether anion gap values in obesity associate with low bicarbonate.

Methods

Data from adult outpatients (n=94,448) in the Bronx, New York were collected from 2010 to 2018. Mixed effects models and Cox proportional hazards models were used to examine associations of BMI with elevated anion gap and anion gap metabolic acidosis and of baseline anion gap with incident low bicarbonate and anion gap metabolic acidosis. Anion gap was defined using traditional and albumin-corrected calculations.

Results

Greater BMI was associated with higher anion gap over time and with progressively greater risk of developing an elevated anion gap (hazard ratio [HR] for body mass index [BMI]≥40 kg/m² versus 18 to <25 kg/m², 1.32; 95% confidence interval [95% CI], 1.23 to 1.42 for traditional and HR for BMI≥40 kg/m² versus 18 to <25 kg/m², 1.74; 95% CI, 1.63 to 1.85 for corrected). Higher BMI was also associated with increased risk of developing anion gap metabolic acidosis (HR for BMI≥40 kg/m², 1.53; 95% CI, 1.39 to 1.69). Among patients with obesity, higher anion gap was associated with increased risk of incident low bicarbonate (HR for fourth versus first quartile, 1.29; 95% CI, 1.23 to 1.44 for traditional and HR for fourth versus first quartile, 1.36; 95% CI, 1.26 to 1.48 for corrected) and higher risk of anion gap metabolic acidosis (HR for fourth versus first quartile, 1.78; 95% CI, 1.59 to 1.99).

Conclusions

Obesity is characterized by unmeasured anion accumulation and acid retention or overproduction. Modest elevations in anion gap among patients with obesity are associated with previously unrecognized anion gap metabolic acidosis.

Diagnostic et Prise en Charge de l’Acidose Métabolique Recommandations formalisées d’experts communes Société de réanimation de langue française (SRLF) – Société française de médecine d’urgence (SFMU)

L’acidose métabolique est un trouble fréquemment rencontré en médecine d’urgence et en médecine intensive réanimation. La littérature s’étant enrichie de nouvelles données concernant la prise en charge de l’acidose métabolique, la Société de Réanimation de Langue Française (SRLF) et la Société Française de Médecine d’Urgence (SFMU) ont élaboré des recommandations formalisées d’experts selon la méthodologie GRADE. Les champs de la stratégie diagnostique, de l’orientation et de la prise en charge thérapeutique ont été traités et vingt-neuf recommandations ont été formulées : quatre recommandations fortes (Grade 1), dix recommandations faibles (Grade 2) et quinze avis d’experts. Toutes ont obtenu un accord fort. L’application des méthodes d’Henderson-Hasselbalch et de Stewart pour le diagnostic du mécanisme de l’acidose métabolique est discutée et un algorithme diagnostique est proposé. L’utilisation de la cétonémie et des lactatémies veineuse et capillaire est également traitée. L’intérêt du pH, de la lactatémie et de sa cinétique pour l’orientation des patients en pré-hospitalier et aux urgences est envisagé. Enfin, les modalités de l’insulinothérapie au cours de l’acidocétose diabétique, les indications de la perfusion de bicarbonate de sodium et de l’épuration extra-rénale ainsi que les modalités de la ventilation mécanique au cours des acidoses métaboliques sévères sont traitées dans la prise en charge thérapeutique.

Diagnosis and management of metabolic acidosis: guidelines from a French expert panel

Metabolic acidosis is a disorder frequently encountered in emergency medicine and intensive care medicine. As literature has been enriched with new data concerning the management of metabolic acidosis, the French Intensive Care Society (Société de Réanimation de Langue Française [SRLF]) and the French Emergency Medicine Society (Société Française de Médecine d’Urgence [SFMU]) have developed formalized recommendations from experts using the GRADE methodology. The fields of diagnostic strategy, patient assessment, and referral and therapeutic management were addressed and 29 recommendations were made: 4 recommendations were strong (Grade 1), 10 were weak (Grade 2), and 15 were experts’ opinions. A strong agreement from voting participants was obtained for all recommendations. The application of Henderson–Hasselbalch and Stewart methods for the diagnosis of the metabolic acidosis mechanism is discussed and a diagnostic algorithm is proposed. The use of ketosis and venous and capillary lactatemia is also treated. The value of pH, lactatemia, and its kinetics for the referral of patients in pre-hospital and emergency departments is considered. Finally, the modalities of insulin therapy during diabetic ketoacidosis, the indications for sodium bicarbonate infusion and extra-renal purification as well as the modalities of mechanical ventilation during severe metabolic acidosis are addressed in therapeutic management.

A systematic review and diagnostic test accuracy meta-analysis of the validity of anion gap as a screening tool for hyperlactatemia

Objective

This systematic review and meta-analysis seeks to determine the validity of the anion gap to screen for hyperlactatemia in critically ill patients. We have previously shown that the anion gap does not predict 31-day and in-hospital mortality in critically ill patients. The present review aims to add confirmatory evidence to identify whether the anion gap is a suitable tool for risk stratification in low-resource countries.

Results

Nine studies reporting on 4504 samples from 2111 patients were included. The anion gap failed to detect hyperlactatemia defined as lactate above 2.5 mmol/l but showed good discriminatory ability for the detection of severe hyperlactatemia defined as lactate over 4 mmol/l. At the 2.5 mmol/l threshold, the anion gap had high specificity but low sensitivity for the detection of hyperlactatemia. A meta-analysis of correlation coefficients yielded high statistical heterogeneity. Therefore, in keeping with our previous findings, the use of the anion gap for risk stratification as an alternative to lactate cannot be recommended. However, the strength of the evidence we have synthesised is adversely affected by the small number of studies included, inconsistency of effect measures and positivity thresholds reported, and selection bias within individual studies. PROSPERO Registration Number: CRD42015016470 (registered on the 4th February 2015).

Electronic supplementary material

The online version of this article (10.1186/s13104-017-2853-9) contains supplementary material, which is available to authorized users.

Quantitative relationships among plasma lactate, inorganic phosphorus, albumin, unmeasured anions and the anion gap in lactic acidosis

BACKGROUND

Quantitative relationships among plasma [Lactate], [Pi], [Albumin], unmeasured anions ([UA]) and the anion gap (AG_K) in lactic acidosis (LA) are not well defined.

METHODS

A mathematical model featuring compensatory potassium and chloride shifts and respiratory changes in LA demonstrated: (1) AG_K=[Lactate]+Zp×[Pi]+2.4×[Albumin]+constant1+e, where Zp is a function of pH, and e reflects unmeasured anions and cations plus pH-related variations. Eq. (1) can be algebraically rearranged to incorporate the albumin-corrected anion gap, cAG_K: (2) cAG_K=[Lactate]+Zp×[Pi]+constant2+e. Eq. (1) was tested against 948 data sets from critically ill patients with [Lactate] 4.0mEq/L or greater. AG_K and cAG_K were evaluated against 12,341 data sets for their ability to detect [Lactate]>4.0mEq/L.

RESULTS

Analysis of Eq. (1) revealed r²=0.5950, p<0.001. cAG_k>15mEq/L exhibited a sensitivity of 93.0% [95% CI: 91.3-94.5] in detecting [Lactate]>4.0mEq/L, whereas AG_K>15mEq/L exhibited a sensitivity of only 70.4% [67.5-73.2]. Additionally, [Lactate]>4.0mEq/L and cAG_K>20mEq/L were each strongly associated with intensive care unit mortality (χ²>200, p<0.0001 for each).

CONCLUSIONS

In LA, cAG_K is more sensitive than AG_K in predicting [Lactate]>4.0mEq/L.

Low sensitivity of anion gap to detect clinically significant lactic acidosis in the emergency department

INTRODUCTION

Lactic acidosis represents the pathologic accumulation of lactate and hydrogen ions. It is important to efficiently diagnose lactic acidosis as delayed treatment will lead to poor patient outcomes. As plasma lactate levels may not be rapidly available, some physicians may use elevated anion gaps to test for the need to measure lactate. All Edmonton metropolitan hospitals have Radiometer blood gas/electrolyte instruments in the ED or close by. As lactate is measured for each set of electrolytes, we were able to determine the effectiveness of a screening anion gap for lactic acidosis.

METHODS

Two years of emergency department lactates and electrolytes from Edmonton's 5 metropolitan hospitals were analyzed. We determined the sensitivity, specificity and positive predictive value of detecting an elevated lactate, defined as ≥2.5mmol/L or ≥4mmol/L.

RESULTS

Depending on the elevated anion gap cut-off and the definition of elevated lactate, between 40-80% of elevated lactates are missed. In general, the positive predictive value approaches 40% for AGs ≥12mmol/L and 60% for AGs ≥16mmol/L.

CONCLUSIONS

Anion gap is an inadequate marker of lactic acidosis. We recommend that lactate be done with each set of electrolytes and/or blood gases. In this way lactic acidosis will not be missed.

Anion gap as a prognostic tool for risk stratification in critically ill patients - a systematic review and meta-analysis

BACKGROUND

Lactate concentration is a robust predictor of mortality but in many low resource settings facilities for its analysis are not available. Anion gap (AG), calculated from clinical chemistry results, is a marker of metabolic acidosis and may be more easily obtained in such settings. In this systematic review and meta-analysis we investigated whether the AG predicts mortality in adult patients admitted to critical care settings.

METHODS

We searched Medline, Embase, Web of Science, Scopus, The Cochrane Library and regional electronic databases from inception until May 2016. Studies conducted in any clinical setting that related AG to in-hospital mortality, in-intensive care unit mortality, 31-day mortality or comparable outcome measures were eligible for inclusion. Methodological quality of included studies was assessed using the Quality in Prognostic Studies tool. Descriptive meta-analysis was performed and the I(2) test was used to quantify heterogeneity. Subgroup analysis was undertaken to identify potential sources of heterogeneity between studies.

RESULTS

Nineteen studies reporting findings in 12,497 patients were included. Overall, quality of studies was poor and most studies were rated as being at moderate or high risk of attrition bias and confounding. There was substantial diversity between studies with regards to clinical setting, age and mortality rates of patient cohorts. High statistical heterogeneity was found in the meta-analyses of area under the ROC curve (I(2) = 99 %) and mean difference (I(2) = 97 %) for the observed AG. Three studies reported good discriminatory power of the AG to predict mortality and were responsible for a large proportion of statistical heterogeneity. The remaining 16 studies reported poor to moderate ability of the AG to predict mortality. Subgroup analysis suggested that intravenous fluids affect the ability of the AG to predict mortality.

CONCLUSION

Based on the limited quality of available evidence, a single AG measurement cannot be recommended for risk stratification in critically ill patients. The probable influence of intravenous fluids on AG levels renders the AG an impractical tool in clinical practice. Future research should focus on increasing the availability of lactate monitoring in low resource settings.

PROSPERO REGISTRATION NUMBER

CRD42015015249 . Registered on 4th February 2015.

Physicochemical Approach to Determine the Mechanism for Acid-Base Disorders in 793 Hospitalized Foals

BACKGROUND

The quantitative effect of strong electrolytes, unmeasured strong anions (UAs), pCO2, and plasma protein concentrations in determining plasma pH can be demonstrated using the physicochemical approach. Plasma anion gap (AG) and strong ion gap (SIG) are used to assess UAs in different species.

HYPOTHESES

Strong ions are a major factor influencing changes in plasma pH of hospitalized foals. AG and SIG accurately predict severe hyper-L-lactatemia ([L-lac(-)] > 7 mmol/L).

ANIMALS

Seven hundred and ninety three hospitalized foals < 7 days old.

METHODS

Retrospective study. The relationship between measured pH and physicochemical variables, and the relationship between plasma [L-lac(-)] and AG and SIG, were determined using regression analyses. Optimal AG and SIG cut points to predict hyper-L-lactatemia were identified using an ROC curve analysis.

RESULTS

Combined, the measured strong ion difference and SIG accounted for 54-69% of the changes in the measured arterial pH of hospitalized foals. AG and SIG were significantly associated with plasma [L-lac(-)] (P < .0001). The receiver operator characteristics (ROC) AUC of AG and SIG for prediction of severe hyper-L-lactatemia were 0.89 (95%CI, 0.8-0.95; P < .0001) and 0.90 (95%CI, 0.81-0.96; P < .0001), respectively. Severe hyper-L-lactatemia was best predicted by AG > 27 mmol/L (sensitivity 80%, 95%CI, 56-94, specificity 85%, 95%CI, 73-93; P < .0001) and SIG <-15 mmol/L (sensitivity 90%, 95%CI, 68-98; specificity 80%; 95%CI, 68-90; P < .0001).

CONCLUSION AND CLINICAL RELEVANCE

Altered concentrations of strong ions (Na(+), K(+), Cl(-)) and UAs were the primary cause of acidemia of hospitalized foals. AG and SIG were good predictors of hyper-L-lactatemia and could be used as surrogate tests.

Effect of the independent acid base variables on anion gap variation in cardiac surgical patients: a Stewart-Figge approach

PURPOSE

To determine the effect of each of independent acid base variables on the anion gap (AG) value in cardiac surgical patients.

METHODS

This retrospective study involved 128 cardiac surgical patients admitted for postoperative care. The variation of AG (AGvar) between the day of admission and the first postoperative day was correlated via a multiple linear regression model with the respective variations of the independent acid base variables, that is, apparent strong ion difference (SIDa), strong ion gap (SIG), carbon dioxide (PCO2), and albumin and phosphate concentrations.

RESULTS

The variations of all the above variables contributed significantly to the prediction of AGvar (adjusted R (2) = 0.9999, F = 201890.24, and P < 0.001). According to the standardized coefficients (β),  SIGvar (β = 0.948, P < 0.001), [Albumin]var (β = 0.260, P < 0.001), and [Phosphate]var (β = 0.191, P < 0.001) were the major determinants of AGvar with lesser contributions from SIDa, var (β = 0.071, P < 0.001) and PCO2, var (β = -0.067, P < 0.001).

CONCLUSIONS

All the independent acid base variables contribute to the prediction of the AG value. However, albumin and phosphate and SIG variations seem to be the most important predictors, while AG appears to be rather stable with changes in PCO2 and SIDa.

The serum anion gap in the evaluation of acid-base disorders: what are its limitations and can its effectiveness be improved?

The serum anion gap has been utilized to identify errors in the measurement of electrolytes, to detect paraproteins, and, most relevant to the nephrologist, to evaluate patients with suspected acid-base disorders. In regard to the latter purpose, traditionally an increased anion gap is identified when it exceeds the upper limit of normal for a particular clinical laboratory measurement. However, because there is a wide range of normal values (often 8-10 mEq/L), an increase in anion concentration can be present in the absence of an increased anion gap. In addition, the type of retained anion can affect the magnitude of the increase in anion gap relative to change in serum [HCO3(-)] being greater with lactic acidosis compared with ketoacidosis. This review examines the methods of calculation of the serum anion gap in textbooks and published literature, the effect of perturbations other than changes in acid-base balance, and its effectiveness in identifying mild and more severe disturbances in acid-base balance. Limitations of the present methods of determining the normal anion gap and change in the anion gap are highlighted. The possibility of identifying the baseline value for individuals to optimize the use of the calculation in the detection of metabolic acidosis is suggested.

Ion-selective electrode and anion gap range: What should the anion gap be?

Background

Using flame photometry technique in the 1970s, the normal value of anion gap (AG) was determined to be 12 ± 4 meq/L. However, with introduction of the autoanalyzers using an ion-selective electrode (ISE), the anion gap value has fallen to lower levels.

Methods

A retrospective study of US veterans from a single medical center was performed to determine the value of the anion gap in subjects with normal renal function and normal serum albumin and in patients with lactic acidosis and end-stage renal disease on dialysis.

Results

In 409 patients with an estimated glomerular filtration rate ≥60 mL/min/1.73 m² body surface area and serum albumin ≥4 g/dL, the mean AG was 7.2 ± 2 (range 3–11) meq/L. In 299 patients with lactic acidosis (lactate level ≥4 meq/L) and 68 patients with endstage renal disease on dialysis, the mean AG was 12.5 meq/L and 12.4 meq/L, respectively. A value <2 meq/L should be considered a low anion gap and a possible clue to drug intoxication and paraproteinemic disorders.

Conclusion

With the advent of ISE for measurement of analytes, the value of the anion gap has fallen. Physicians need to be aware of the normal AG value in their respective institutions, and laboratories need to have an established value for AG based on the type of instrument they are using.

The difference between critical care initiation anion gap and prehospital admission anion gap is predictive of mortality in critical illness

OBJECTIVE

We hypothesized that the delta anion gap defined as difference between critical care initiation standard anion gap and prehospital admission standard anion gap is associated with all cause mortality in the critically ill.

DESIGN

Observational cohort study.

SETTING

Two hundred nine medical and surgical intensive care beds in two hospitals in Boston, MA.

PATIENTS

Eighteen thousand nine hundred eighty-five patients, age ≥18 yrs, who received critical care between 1997 and 2007.

MEASUREMENTS

The exposure of interest was delta anion gap and categorized a priori as <0, 0-5, 5-10, and >10 mEq/L. Logistic regression examined death by days 30, 90, and 365 postcritical care initiation and in-hospital mortality. Adjusted odds ratios were estimated by multivariable logistic regression models. The discrimination of delta anion gap for 30-day mortality was evaluated using receiver operator characteristic curves performed for a subset of patients with all laboratory data required to analyze the data via physical chemical principles (n = 664).

INTERVENTIONS

None.

RESULTS

Delta anion gap was a particularly strong predictor of 30-day mortality with a significant risk gradient across delta anion gap quartiles following multivariable adjustment: delta anion gap <0 mEq/L odds ratio 0.75 (95% confidence interval 0.67-0.81; p < 0.0001); delta anion gap 5-10 mEq/L odds ratio 1.56 (95% confidence interval 1.35-1.81; p < 0.0001); delta anion gap >10 mEq/L odds ratio 2.18 (95% confidence interval 1.76-2.71; p < 0.0001); and all relative to patients with delta anion gap 0-5 mEq/L. Similar significant robust associations post multivariable adjustments are seen with death by days 90 and 365 as well as in-hospital mortality. Correcting for albumin or limiting the cohort to patients with standard anion gap at critical care initiation of 10-18 mEq/L did not materially change the delta anion gap-mortality association. Delta anion gap has similarly moderate discriminative ability for 30-day mortality in comparison to standard base excess and strong ion gap.

CONCLUSION

An increase in standard anion gap at critical care initiation relative to prehospital admission standard anion gap is a predictor of the risk of all cause patient mortality in the critically ill.

The Stewart approach--one clinician's perspective

Peter Stewart added controversy to an already troubled subject when he entered the clinical acid-base arena. His approach puts water dissociation at the centre of the acid-base status of body fluids. It is based on six simultaneous equations, incorporating the Laws of Mass Action, Mass Conservation, and Electrical Neutrality. Together with Gibbs-Donnan equilibria, these equations explain the diagnostically important PaCO(2)/pH relationship, and improve understanding of the physiologic basis of traditional acid-base approaches. Spin-offs have included new scanning tools for unmeasured ions, in particular the 'strong ion gap' and 'net unmeasured ions'. The most controversial feature is the designation of pH and bicarbonate concentrations as dependent variables, answerable exclusively to three independent variables. These are the strong ion difference (SID), the total concentration of non-volatile weak acid (A(TOT)), and PCO(2). Aspects of this assertion conflict with traditional renal physiology, and with current models of membrane H(+)/base transporters, oxidative phosphorylation, and proton and bicarbonate ionophores. The debate in this area is ongoing. Meanwhile, Stewart-style diagnostic and decision support systems such as the 'Strong Ion Calculator' and the web-site www.acidbase.org are now appearing.

Anion gap, anion gap corrected for albumin, base deficit and unmeasured anions in critically ill patients: implications on the assessment of metabolic acidosis and the diagnosis of hyperlactatemia

Background

Base deficit (BD), anion gap (AG), and albumin corrected anion gap (ACAG) are used by clinicians to assess the presence or absence of hyperlactatemia (HL). We set out to determine if these tools can diagnose the presence of HL using cotemporaneous samples.

Methods

We conducted a chart review of ICU patients who had cotemporaneous arterial blood gas, serum chemistry, serum albumin (Alb) and lactate(Lac) levels measured from the same sample. We assessed the capacity of AG, BD, and ACAG to diagnose HL and severe hyperlactatemia (SHL). HL was defined as Lac > 2.5 mmol/L. SHL was defined as a Lac of > 4.0 mmol/L.

Results

From 143 patients we identified 497 series of lab values that met our study criteria. Mean age was 62.2 ± 15.7 years. Mean Lac was 2.11 ± 2.6 mmol/L, mean AG was 9.0 ± 5.1, mean ACAG was 14.1 ± 3.8, mean BD was 1.50 ± 5.4. The area under the curve for the ROC for BD, AG, and ACAG to diagnose HL were 0.79, 0.70, and 0.72, respectively.

Conclusion

AG and BD failed to reliably detect the presence of clinically significant hyperlactatemia. Under idealized conditions, ACAG has the capacity to rule out the presence of hyperlactatemia. Lac levels should be obtained routinely in all patients admitted to the ICU in whom the possibility of shock/hypoperfusion is being considered. If an AG assessment is required in the ICU, it must be corrected for albumin for there to be sufficient diagnostic utility.

Acid-base analysis: a critique of the Stewart and bicarbonate-centered approaches

When approaching the analysis of disorders of acid-base balance, physical chemists, physiologists, and clinicians, tend to focus on different aspects of the relevant phenomenology. The physical chemist focuses on a quantitative understanding of proton hydration and aqueous proton transfer reactions that alter the acidity of a given solution. The physiologist focuses on molecular, cellular, and whole organ transport processes that modulate the acidity of a given body fluid compartment. The clinician emphasizes the diagnosis, clinical causes, and most appropriate treatment of acid-base disturbances. Historically, two different conceptual frameworks have evolved among clinicians and physiologists for interpreting acid-base phenomena. The traditional or bicarbonate-centered framework relies quantitatively on the Henderson-Hasselbalch equation, whereas the Stewart or strong ion approach utilizes either the original Stewart equation or its simplified version derived by Constable. In this review, the concepts underlying the bicarbonate-centered and Stewart formulations are analyzed in detail, emphasizing the differences in how each approach characterizes acid-base phenomenology at the molecular level, tissue level, and in the clinical realm. A quantitative comparison of the equations that are currently used in the literature to calculate H+concentration ([H+]) is included to clear up some of the misconceptions that currently exist in this area. Our analysis demonstrates that while the principle of electroneutrality plays a central role in the strong ion formulation, electroneutrality mechanistically does not dictate a specific [H+], and the strong ion and bicarbonate-centered approaches are quantitatively identical even in the presence of nonbicarbonate buffers. Finally, our analysis indicates that the bicarbonate-centered approach utilizing the Henderson-Hasselbalch equation is a mechanistic formulation that reflects the underlying acid-base phenomenology.