Acid Base Made Easy

(+)Scott C. Sherman, MD
Associate Professor of Emergency
Medicine, Rush Medical College;
Assistant Program Director, Cook County
Emergency Medicine Residency,
Chicago, Illinois

Acid-Base Made Easy
The differential diagnoses for acid-base problems can be
reduced to a workable few by using a minimal amount of
laboratory data. Following well-established principles and
formulas, the presenter will help you resolve common acid-
base problem cases.
• Identify etiologies of anion gap and non-anion gap
acidosis.
• Explain the principle of osmolar gap.
• Differentiate the causes of acid-base disturbance and
discuss appropriate management.
• Identify potential life-threatening disorders by working
through real ED cases.

WE-248
Wednesday, October 7, 2009
5:00 PM - 5:50 PM
Boston Convention & Exhibition Center

(+)No significant financial relationships to disclose

Acid-Base Made Easy
Scott C Sherman, MD
Assistant Residency Director
Department of Emergency Medicine
Cook County Hospital (Stroger)
Assistant Professor of Emergency Medicine
Rush Medical College

Acid-base analysis strikes fear into the minds of both seasoned clinicians and their junior
counterparts. Multiple formulas and rules exist to help guide us through the forest of
diagnoses and complex problems. This lecture is set-up to provide a simple, systematic
approach to interpreting arterial blood gas (ABG) samples. All that is needed is a little
clinical information obtained from a history and physical examination, a few readily
available laboratory tests, and the knowledge of five simple steps. Getting in the routine
of performing these steps on each patient in which an ABG and electrolytes are
performed will help decrease the rate of missed complex acid-base disturbances and
hopefully improve patient care.

Five Steps of Acid-Base Analysis1-5

Step 1: Acidemia (pH <7.38) or alkalemia (pH >7.42)?

Step 2: Primary respiratory or metabolic disturbance?
(Look at PCO2 on ABG or HCO3 on metabolic panel.)

Step 3: Is there appropriate compensation for the primary disorder?
Metabolic acidosis: PCO2 = [1.5 x (serum HCO3)] + 8 (±2)
Metabolic alkalosis: ↑PCO2 = 0.6 x ↑HCO3 (±2)
Respiratory acidosis: ↑PCO2 10, ↑ HCO3 by 1 (acute) or 4 (chronic)
Respiratory alkalosis: ↓PCO2 10, ↓ HCO3 by 2 (acute) or 5 (chronic)

Step 4: Is there an anion gap metabolic acidosis (AGMA)?
AG = Na - (HCO3 + Cl). If > 12, an AGMA is present.

Step 5: If metabolic acidosis, is there another concomitant metabolic disturbance?
If AGMA, then calculate ∆Gap = ∆AG – ∆ HCO3 = (AG -12) – (24 – HCO3)
If the ∆Gap is > 6, there is a combined AGMA and metabolic alkalosis.
If the ∆Gap is < -6, there is a combined AGMA and NAGMA.
If NAGMA, for every 1 mEq/L ↑Cl, there should be a 1 mEq/L ↓ HCO3 (±5).
If HCO3 decrease is less than predicted, then NAGMA and metabolic
alkalosis

1

Explanation of the Five Steps
Step 1. This step is straightforward. Look at the pH. Is the blood acidemic or alkalemic?
This is the primary disorder. Any compensation for a metabolic disturbance by the lungs
or vice versa will not bring the pH back to “normal”.

Step 2. Determine whether the primary disorder is respiratory or metabolic. This is
accomplished by looking at the bicarbonate on the chemistry or the pCO2 on the ABG. In
acidemia, low bicarbonate (< 24) and low pCO2 (< 40) suggests a metabolic acidosis.
Alternatively, a high bicarbonate (> 24) and high pCO2 (> 40) suggests that the primary
disorder is a respiratory one. The opposite is true for alkalemia. A patient with an
elevated bicarbonate (> 24) and pCO2 (> 40) supports a metabolic alkalosis, while low
bicarbonate (< 24) and low pCO2 (<40) supports a respiratory alkalosis.

Step 3. The next question you would like to answer is whether or not the other body
system (kidneys in a primary respiratory disorder or lungs in a primary metabolic
disorder) are compensating appropriately.
• Metabolic acidosis: Whether compensation is adequate or not is easiest to answer
when the primary disorder is a metabolic acidosis. In this case, Winter’s formula
is used. Winter’s formula states that the patient’s pCO2 should be equal to the
serum bicarbonate multiplied by 1.5 plus eight6. When this number is within two
of the pCO2, the respiratory system is compensating appropriately. PCO2 =
1.5(HCO3)+8 ±2. If the patient’s pCO2 is higher than expected, a respiratory
acidosis is present in addition to the primary metabolic acidosis. If the patient’s
pCO2 is less than expected, then there is a respiratory alkalosis in addition to the
primary metabolic acidosis.
• Metabolic alkalosis: The respiratory system compensates for a metabolic alkalosis
by increasing the pCO2 level. However, unlike a metabolic acidosis, the normal
respiratory compensation to a metabolic alkalosis is difficult to predict and the
pCO2 level rarely rises above 50 mmHg. However, the increase in the pCO2 is
approximately equal to the increase in HCO3 multiplied by 0.61.
• Respiratory acidosis/alkalosis: Renal compensation for respiratory acid-base
abnormalities improves with time. Acute changes (48-72 hours) in respiratory
function occur due to titration of bicarbonate by available buffer systems. In
chronic situations (> 72 hours), the kidney is able to alter production and
resorption of bicarbonate and ultimately affect a larger change. In acute
respiratory acidosis, for every pCO2 increase of 10 mmHg, bicarbonate increases
by 1 mEq/L. In chronic respiratory acidosis, a similar change in pCO2 will result
in a bicarbonate change of 4 mEq/L. In patients with acute respiratory alkalosis, a
pCO2 decrease of 10 mmHg produces a drop in bicarbonate of 2 mEq/L, and
when chronic, the bicarbonate drops 5 mEq/L. These formulas will give the
clinician a rough estimate of whether the respiratory acid-base disorder is being
compensated for by the kidneys appropriately and whether the patient is suffering
from an acute, chronic, or acute-on-chronic respiratory ailment.

2

Step 4. Calculate the anion gap. This step must be carried out on all patients, not just
those with a primary metabolic acidosis. The presence of an anion gap, with rare
exceptions, means that an anion gap metabolic acidosis is present7. The anion gap is
calculated by subtracting the sum of the chloride and bicarbonate from the sodium level.
Na – (Cl + HCO3). For the purposes of this lecture and the sample calculations within it,
the normal anion gap will be assumed to be 12. In reality, it is most likely slightly lower
than this (approximately 6-10), but it varies from individual to individual and the most
accurate determination would be a baseline anion gap before the patient became ill.

Step 5. This step is useful to detect a previously undetected metabolic disorder not
discovered on the previous four steps. The premise is that for every increase in positive
charges, there must be an equal decrease in negative charges. Therefore, in a patient with
an anion gap metabolic acidosis, the elevation of the anion gap over 12 should be
accompanied by a similar decrease in the bicarbonate concentration from normal (ie, 24).
If the bicarbonate is lower than expected, then an additional metabolic acidosis of the
non-anion gap variety exists (this may be a primary NAGMA or compensation for
respiratory alkalosis as discovered in step 3). If the bicarbonate concentration is more
than expected, then a metabolic alkalosis is present8. This calculation is known as the
“delta gap” or sometimes the “delta-delta”. To avoid over-diagnosis and to account for
the normal range of values for the anion gap and bicarbonate levels, a delta gap of greater
than 6 or less than -6 is considered significant7,9. This range accounts for two standard
deviations around the mean7.
In patients with a non-anion gap metabolic acidosis, a previously undetected
metabolic alkalosis can be detected by noting that for every 1 mEq/L increase in the
chloride (from the normal of 100; hyperchloremic acidosis) there should be a similar 1
mEq/L decrease in the bicarbonate level from 24. If the bicarbonate decrease is less than
expected (by five to account for two standard deviations from the mean), then an
underlying metabolic alkalosis is also diagnosed1.

Once the acid-base disturbances have been determined using the above steps, the finite
list of clinical conditions that cause them should be considered based on the patient’s
medical history. As with any diagnostic test, the results of acid-base determinations
cannot be properly interpreted without clinical correlation10.

• Respiratory Acidosis
Respiratory acidosis is characterized by an elevation in the pCO2 and a decrease in blood
pH due most commonly to hypoventilation. It results from conditions that decrease the
ability of the lungs to excrete carbon dioxide at a rate to keep up with the body’s
production. A differential diagnosis includes:

o Central nervous system depression (sedatives, CNS disease, sleep apnea)
o Pleural disease (large pneumothorax or pleural effusion)

3

o Lung disease (ARDS, COPD, pulmonary edema, severe pneumonia)
o Acute airway obstruction (laryngospasm, sleep apnea)
o Neuromuscular disorders (GBS, myasthenia gravis, botulism)
o Thoracic cage injury (flail chest)
o Ventilator dysfunction

The kidney compensates for primary respiratory acidosis by retaining bicarbonate. This
compensation occurs over hours to days and is generally at a maximum within four days.
The rate of onset of respiratory acidosis can be determined by the degree of renal
compensation (increase in HCO3) as listed above in step 3. Alternatively, the chronicity
of the respiratory acidosis can be predicted by the change in the pH. In acute respiratory
acidosis, the pH decreases by 0.08 units for each increase of 10 mmHg in the pCO2 from
its baseline of 40 mmHg. Chronic respiratory acidosis is marked by a decrease in the pH
of 0.03 units for every increase of 10 mmHg in the pCO2.

Differentiating acute from chronic respiratory conditions can have important clinical
implications that may alert the clinician to a patient that is rapidly spiraling downward
and might require emergent intubation, from a patient who has chronic disease, but is in
less danger of imminent decompensation5.

Example: A 70 year-old smoker presents with an acute onset of shortness of breath.
ABG: pH 7.30, pCO2 = 60 mmHg, pO2 60 mmHg
Metabolic panel: Na 135, Cl 100, HCO3 30

Step 1: Acidosis
Step 2: Respiratory
Step 3: Acute on chronic. pCO2 increased by 20, therefore the HCO3 should increase by 2
if acute and 8 if chronic. Because the HCO3 increased from 24 to 30 (6), an acute on
chronic respiratory acidosis is present.
Step 4: AG = 135-(100+26) = 9. No anion gap metabolic acidosis
Step 5: XX

Answer: Acute on chronic respiratory acidosis due to COPD exacerbation

• Respiratory Alkalosis
In contrast to respiratory acidosis, respiratory alkalosis is characterized by a decrease in
the pCO2 and an elevation in the blood pH. The pO2 can be used to distinguish between
disease of the lungs and other causes of hyperpnea (eg, fever)11. The causes of a primary
respiratory alkalosis include:

o CNS disease (CVA)
o Toxins (Salicylates)
o High altitude

4

o Severe anemia
o Pregnancy
o Lung disease/hypoxia (asthma, pneumonia, PE, pulmonary edema, pulmonary
fibrosis)
o Anxiety
o Cirrhosis of the liver
o Fever (Sepsis)
o Ventilator dysfunction

Example: A 22 year-old woman presents with 4 hours of numbness in both hands typical
of previous episodes of anxiety.
ABG: pH 7.48, pCO2 30 mmHg, pO2 86 mmHg

Step 1: Alkalosis
Step 2: Respiratory
Step 3: Acute. Drop in the pCO2 by 10 corresponds to a drop in the HCO3 by 2 if acute
and 5 if chronic. 24-22 = 2 and therefore, as would be expected by the clinical history, an
acute disorder is diagnosed. Note also that the pH went up by 0.08 as would be expected
in an acute condition.
Step 4: AG 140-(110+22) = 8
Step 5: XX

Answer: Acute respiratory alkalosis secondary to a panic attack

• Metabolic Acidosis
In the presence of a pH < 7.38, metabolic acidosis is diagnosed as a primary condition
when the pCO2 is < 40 mmHg or the bicarbonate is < 24 mEq/L. Metabolic acidosis can
be further classified based on the presence of an anion gap. The anion gap reflects the
balance between positively and negatively charged particles in the blood. Sodium is the
only significant positively charge particle that is measured, while the measured anions are
chloride and bicarbonate. Therefore, the anion gap is calculated by the formula: Na – (Cl
+ HCO3).

A “normal” anion gap is 6-12, although based on newer techniques for measurement
using ion selective electrodes, the normal anion gap is approximately 6 (range 3-11)12.
Unmeasured anions are negatively charged proteins (15 mEq/L), phosphates (2 mEq/L),
sulfates (1 mEq/L), and organic acids (5 mEq/L); for a total of 23 mEq/L. Unmeasured
cations include potassium (4.5 mEq/L), calcium (5mEq/L), and magnesium (1.5 mEq/L);
for a total of 11 mEq/L.

One potential pitfall in the measurement of the anion gap is patients with low albumin.
Albumin has several negative charges on it and therefore, in a patient with a low albumin

5

level, their “normal” anion gap might be much lower than 12. For every 1 gram drop in
serum albumin level, the anion gap decreases by 2.51. A patient with a calculated anion
gap of 10 and a 2 gram drop in their albumin may actually have an anion gap metabolic
acidosis (recalculated AG 15).

Anion Gap Metabolic Acidosis (AGMA)
Detection of an AGMA is important because only a few conditions commonly cause it13.
In addition, in mixed acid-base disorders, an elevation in the anion gap may be the only
signal that a metabolic acidosis is present. The causes of an AGMA are divided into four
main categories: renal failure, ketoacidosis, toxins, and lactic acidosis. Causes of lactic
acidosis include conditions with inadequate tissue perfusion (hypoxemia, low cardiac
output, peripheral shunting, sepsis).

A CAT MUDPILES
o Analgesics (massive NSAID, acetaminophen)
o Cyanide, Carbon monoxide
o Arsenic, Alcoholic ketoacidosis
o Toluene
o Methanol, Metformin
o Uremia
o Diabetic ketoacidosis
o Paraldehyde, Phenformin
o Iron, Isoniazid
o Lactic acidosis
o Ethylene glycol
o Salicylates

An anion gap of 20 or more suggests that metabolic acidosis is present regardless of the
pH or serum bicarbonate level. This statement is accurate because the body does not
compensate for alkalosis by generating a large anion gap. The degree of rise in the anion
gap also suggests how likely the cause will be diagnosed. In one study, when the anion
gap was between 12-20, the specific cause was found in < 30% of patients. In patients
with an anion gap > 20, the cause was identified in 77%, and when the gap was > 30,
100% of patients were diagnosed14.

Patients with alcoholic ketoacidosis will have an AGMA with ketones in their blood and
urine. Because alcoholics can present with a multitude of problems (DT’s, infection,
vomiting, dehydration, pancreatitis) their acid-base abnormalities may be quite complex
and require careful consideration. Although the pH is most frequently low, it can also be
elevated or normal. The pathogenesis is related to both the effects of ethanol and acute
starvation from lack of any other caloric intake. The metabolism of ethanol provides
substrate for ketone production and decreases gluconeogenesis15.

In any patient with an AGMA, calculate an osmol gap. Osmol gaps are a clue to a
potentially life-threatening toxic alcohol ingestion (ie. ethylene glycol and methanol).

6

The osmol gap is determined by subtracting the calculated osmolality from the measured
osmolality. Calculated osmolality = 2(Na) + Glc/18 + BUN/2.4 + ETOH/4.6. When the
measured osmolarity is more than the calculated osmolarity by 10 mOsm/kg H2O, then
the presence of an unmeasured substance should be considered16. The mnemonic for
remembering the clinically important substances that cause an osmol gap is ME LIE
(methanol, ethylene glycol, lactate, isopropyl alcohol, and ethanol)17. Unfortunately, a
normal osmol gap does not exclude the possibility of a toxic alcohol ingestion18.

Example: 32 year old man with depression and alcohol abuse presents with altered
mental status.
ABG: pH 6.9, pCO2 29, pO2 100

Step 1: Acidosis
Step 2: Metabolic
Step 3: pCO2 = 1.5(HCO3) + 8 = 15, but the patient’s pCO2 is higher than 15. Therefore,
a respiratory acidosis is also present, possibly secondary to CNS depression.
Step 4: AG = 140 – (101 + 5) = 34
Step 5: Delta gap = (34-12) – (24-5) = 3. No additional metabolic disorders other than
AGMA.

Answer: Anion gap metabolic acidosis and respiratory acidosis. The patient had an
osmol gap of 174 and a methanol level of 510 mg/dL.

Non-Anion Gap Metabolic Acidosis (NAGMA)
A NAGMA is due to either GI or renal losses of bicarbonate. If desired, GI mediated and
renally mediated losses can be distinguished by obtaining urine electrolytes (ie. Na, K,
and Cl) and calculating the urine anion gap. The urine anion gap is the difference
between the spot urine positive ions and spot urine negative ions. If an excess of
negatively charged ions is present, the acidemia is due to the kidney1. Examples of
NAGMAs include11:
o GI bicarbonate losses (diarrhea).
o Renal tubular acidosis.
o Carbonic anhydrase inhibitors.
o Ureteral diversions.
o Rehydration.
o K sparing diuretics
o NSAIDs
o Addison’s disease

Example: A 68 year old man who recently took antibiotics for a skin infection presents
with 10 episodes of watery diarrhea per day for the last 5 days.
ABG: pH 7.34, pCO2 34, pO2 80
Step 1: Acidosis

7

Step 2: Metabolic
Step 3: pCO2 = 1.5(HCO3) + 8 = 35
Step 4: AG = 135 – (108 + 18) = 9
Step 5: Cl ↑ by 8 and HCO3 ↓ by 6; therefore there is no metabolic alkalosis.

Answer: NAGMA due to diarrhea

• Metabolic Alkalosis
Metabolic alkalosis is characterized by an increase in the serum bicarbonate
concentration. The most common cause of metabolic alkalosis is volume contraction.
This may come in the form of obvious losses such as vomiting, but also occurs with
significant third spacing of fluids. The mechanism of contraction alkalosis is related to
increased mineralocorticoid secretion in response to extracellular volume depletion.
Renal sodium resorption is enhanced in exchange for increased potassium and hydrogen
ion excretion. Hypokalemia is a frequent finding in patients with metabolic alkalosis due
to vomiting or diuretic use, but its presence also promotes metabolic alkalosis. The
mechanism involves a transcellular shift of potassium out of the cell. Hydrogen ion enters
the cell and this effect in the renal tubular cells results in increased hydrogen ion
secretion and increased plasma bicarbonate. The causes of metabolic alkalosis are listed
below.
o Volume contraction (vomiting, NG suction, loop or thiazide diuretics).
o Excess glucocorticoids or mineralocorticoids (eg, Cushing’s syndrome).
o Hypokalemia
o Bartter’s syndrome.
o Alkali ingestion/infusion.
o Post-hypercapnic alkalosis

Example: A 20 year old student presents with excessive vomiting after binge drinking.
ABG: pH 7.50, pCO2 44, pO2 100

Step 1: Alkalosis
Step 2: Metabolic
Step 3: Increase in pCO2 should equal 0.6 multiplied by the elevation of the HCO3 ±2.
The increase of the pCO2 of 4 is within two of 6(0.6) or 3.6; therefore there is
appropriate compensation.
Step 4: AG = 138 – (100 + 30) = 8
Step 5: XX

Answer: Metabolic alkalosis secondary to vomiting.

8

Practice Cases2,7
CASE 1
A diabetic presents with diarrhea and cough. CXR reveals an infiltrate.
pH 7.31; pCO2 10
Na 123; Cl 99; HCO3 5

Step 1: Acidemic or Alkalemic

Step 2: Primary respiratory or metabolic

Step 3: Compensation appropriate?
YES or NO
If met acidosis, then 1.5(HCO3) + 8 = _____±2 = pCO2?
If NO, respiratory acidosis or respiratory alkalosis

Step 4: Na – (Cl + HCO3) = AGMA?
YES or NO

Step 5: If metabolic acidosis, other metabolic disturbances?
If AGMA, calculate delta gap. (AG-12) – (24- HCO3) = _____
> 6, then AGMA and metabolic alkalosis
< -6, then AGMA and NAGMA

CASE 2
An alcoholic presents with vomiting.
pH 7.20; pCO2 25
Na 130; Cl 80; HCO3 10



YES or NO

YES or NO


9

CASE 3
A man with arthritis presents with confusion, shortness of breath, and diaphoresis.
pH 7.30; pCO2 18
Na 147; Cl 108; HCO3 16



YES or NO

YES or NO


CASE 4
A patient with COPD presents with shortness of breath.
pH 7.18; pCO2 80
Na 135; Cl 93; HCO3 30



↑PCO2 10, ↑ HCO3 by 1 (acute) or 4 (chronic)
Acute or Chronic or Both

YES or NO

xxx

10

CASE 5
A woman with Crohn’s disease presents with fever, vomiting, and diarrhea.
pH 7.36; pCO2 22
Na 147; Cl 121; HCO3 14



YES or NO

YES or NO

If NAGMA, increase in Cl should equal decrease in HCO3
If HCO3 decrease is less than predicted, then metabolic alkalosis

CASE 6
A noncompliant patient with diabetes and cirrhosis presents with vomiting.
pH 7.46; pCO2 17
Na 133; Cl 84; HCO3 15



↓PCO2 10, ↓ HCO3 by 2 (acute) or 5 (chronic)
Acute or Chronic or Both

YES or NO


11

Answers:
Case 1: Primary AGMA (DKA), respiratory alkalosis (pneumonia), NAGMA (diarrhea)
Case 2: Primary AGMA (alcoholic ketoacidosis), metabolic alkalosis (vomiting)
Case 3: Primary AGMA and respiratory alkalosis (Salicylate toxicity—107 mg/dl)
Case 4: Primary respiratory acidosis—acute-on-chronic (COPD exacerbation)
Case 5: Primary NAGMA (diarrhea), respiratory alkalosis (fever), metabolic alkalosis (vomiting)
Case 6: Primary chronic respiratory alkalosis (cirrhosis), AGMA (DKA), metabolic alkalosis
(vomiting)

Reference List

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3. Morganroth M. An analytic approach to diagnosing acid-base disorders. The
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6. Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base
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11. Fulop M. Flow diagrams for the diagnosis of acid-base disorders. J Emerg Med
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12. Winter SD, Pearson JR, Gabow PA, Schultz AL, Lepoff RB. The fall of the serum
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13. Emmett M, Narins RG. Clinical use of the anion gap. Medicine (Baltimore) 1977;
56(1):38-54.

14. Gabow PA, Kaehny WD, Fennessey PV, Goodman SI, Gross PA, Schrier RW.
Diagnostic importance of an increased serum anion gap. N Engl J Med 1980;
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15. Fulop M. Alcoholic ketoacidosis. Endocrinol Metab Clin North Am 1993;
22(2):209-219.

16. Church AS, Witting MD. Laboratory testing in ethanol, methanol, ethylene glycol,
and isopropanol toxicities. J Emerg Med 1997; 15(5):687-692.

17. Trummel J, Ford M, Austin P. Ingestion of an unknown alcohol. Ann Emerg Med
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