 # 5 Easy Steps to Handle Arterial Blood Gases (ABGs) on the ABIM Board Exam

One of the seemingly challenging concepts to master for the ABIM Board Exam are arterial blood gases (ABGs). However, by following a simple yet systematic approach, we can tackle these questions without difficulty.

Step #1: Identify Acidemia versus Alkalemia

• Normal pH is 7.35 – 7.45
• If pH < 7.35 : Acidemia is present
• If pH > 7.45 : Alkalemia is present

Step #2: Identify the Primary Change
• If pCO2 is high and pH is low : Primary problem is Respiratory Acidosis
• If pCO2 is low and pH is high : Primary problem is Respiratory Alkalosis
• If HCO3 is low and pH is low : Primary problem is Metabolic Acidosis
• If HCO3 is high and pH is high : Primary problem is Metabolic Alkalosis

Step #3: Check for Compensation
• If the primary problem is Metabolic Acidosis, determine if the body is appropriately compensating with a Respiratory Alkalosis using Winter’s Formula:
pCO2 = 1.5 (HCO3) + 8 +/- 2
• If the primary problem is Metabolic Alkalosis, determine if the body is appropriately compensating with a Respiratory Acidosis using the equation:
pCO2 = 40 + 0.7 (Δ HCO3)
• If the primary problem is Respiratory Acidosis, determine if the body is appropriately compensating with a Metabolic Alkalosis using the following rules.
• Acute: 1 unit increase in HCO3 for every 10 unit increase in pCO2
• Chronic: 3.5 unit increase in HCO3 for every 10 unit increase in pCO2
• If the primary problem is Respiratory Alkalosis, determine if the body is appropriately compensating with a Metabolic Acidosis using the following rules.
• Acute: 2 unit decrease in HCO3 for every 10 unit decrease in pCO2
• Chronic: 5 unit decrease in HCO3 for every 10 unit decrease in pCO2

Step #4: Calculate the Anion Gap
• Anion Gap = Na – (Cl + HCO3)
• Normal Anion Gap = 8 – 12 mEq/L
• If Anion Gap is high and the patient has Metabolic Acidosis, the patient has Widened Anion Gap Metabolic Acidosis (WAGMA).

Causes of WAGMA can be remembered by the mnemonic “MUD PILES”
• Methanol
• Uremia
• Diabetic Ketoacidosis
• Paraldehyde
• Isoniazid
• Lactic Acidosis
• Ethylene Glycol
• Salicylates
NOTE: Salicyclates can also cause Respiratory Alkalosis

If the patient has Metabolic Acidosis and Anion Gap is normal, then the patient has a Normal Anion Gap Metabolic Acidosis (NAGMA).

Two main causes of NAGMA are:
• Diarrhea, which will have a negative urine anion gap
• (Urine Na + Urine Potassium) – Urine Chloride

• Renal tubular acidosis, which will have a positive urine anion gap

Step #5: Check a Delta Gap, if applicable
• If the Anion Gap is greater than 20mEq/L, then a Delta Gap needs to be calculated to determine if additional metabolic disorders are present.
• Delta Gap = Anion Gap – 12
• If Delta Gap + HCO3 > 30, the patient also has Metabolic Alkalosis present
• If Delta Gap + HCO3 < 23, the patient also has NAGMA present

This is a general approach for tackling ABGs, as relevant for your preparation of the ABIM Internal Medicine board exam.