High anion gap metabolic acidosis vs Normal anion gap metabolic acidosis
How to tell High anion gap metabolic acidosis and Normal anion gap metabolic acidosis apart when a board question makes them look alike.
High-gap means added acid; normal-gap means bicarbonate loss or renal acid-handling failure with chloride replacement.
Side by side
Metabolic acidosis with elevated anion gap from added unmeasured acids such as lactate, ketoacids, renal failure acids, or toxins.
Metabolic acidosis with normal anion gap and hyperchloremia, usually from bicarbonate loss or impaired renal acid secretion.
Organic or inorganic acids accumulate, consuming bicarbonate while leaving unmeasured anions behind.
Bicarbonate falls but chloride rises to preserve electroneutrality, so the anion gap stays normal.
Lactic acidosis, ketoacidosis, renal failure, methanol, ethylene glycol, salicylates, or high anion gap points high-gap metabolic acidosis.
Diarrhea, renal tubular acidosis, ureteral diversion, acetazolamide, or hyperchloremia with normal gap points normal-gap metabolic acidosis.
The trap
Low bicarbonate alone does not pick the cause. Calculate the anion gap = Sodium minus (Chloride plus Bicarbonate); normal is 8 to 12 mEq/L. HIGH ANION GAP metabolic acidosis (above 12) means added unmeasured acid, remembered as MUDPILES: Methanol, Uremia, DKA (and starvation/alcoholic ketoacidosis), Propylene glycol (lorazepam IV diluent), Iron and INH, Lactic acidosis (sepsis, shock, metformin, cyanide, carbon monoxide), Ethylene glycol, Salicylates. NORMAL ANION GAP metabolic acidosis (gap normal, hyperchloremia compensates for bicarbonate loss) is HARDASS: Hyperalimentation/parenteral nutrition, Addison disease, Renal tubular acidosis, Diarrhea (most common cause), Acetazolamide, Spironolactone, Saline infusion. Once high-gap is confirmed, calculate the delta-delta (change in gap divided by change in bicarbonate) to detect a coexisting normal-gap acidosis or metabolic alkalosis.
Watch it explained
The repair move
For metabolic acidosis, calculate the gap, then ask toxin/keto/lactate/renal failure versus diarrhea/RTA.
Try a free question
CoreA patient has metabolic acidosis with low bicarbonate and an anion gap of 26 after ethylene glycol ingestion. Which category is most likely?
Show answer and explanation
Answer: A. High anion gap metabolic acidosis. High anion gap metabolic acidosis reflects addition of unmeasured acids such as lactate, ketoacids, toxins, or renal failure acids.
Why the other choices are tempting
- B. Normal anion gap acidosis is usually bicarbonate loss or impaired renal acid secretion with chloride compensation.
- C. Respiratory acidosis is primary CO2 retention.
- D. Metabolic alkalosis has elevated bicarbonate.
- E. Respiratory alkalosis is primary low CO2.
Flashcards
Tap a card to flip it.
What is the fastest separator for High anion gap metabolic acidosis vs Normal anion gap metabolic acidosis?
High-gap means added acid; normal-gap means bicarbonate loss or renal acid-handling failure with chloride replacement.
What clue should make you choose High anion gap metabolic acidosis?
Metabolic acidosis with elevated anion gap from added unmeasured acids such as lactate, ketoacids, renal failure acids, or toxins.
Quick answers
What is the fastest way to tell High anion gap metabolic acidosis from Normal anion gap metabolic acidosis?
High-gap means added acid; normal-gap means bicarbonate loss or renal acid-handling failure with chloride replacement.
What is the key clue for High anion gap metabolic acidosis?
Metabolic acidosis with elevated anion gap from added unmeasured acids such as lactate, ketoacids, renal failure acids, or toxins.
What is the key clue for Normal anion gap metabolic acidosis?
Metabolic acidosis with normal anion gap and hyperchloremia, usually from bicarbonate loss or impaired renal acid secretion.
What is the common trap with High anion gap metabolic acidosis vs Normal anion gap metabolic acidosis?
Low bicarbonate alone does not pick the cause. Calculate the anion gap = Sodium minus (Chloride plus Bicarbonate); normal is 8 to 12 mEq/L. HIGH ANION GAP metabolic acidosis (above 12) means added unmeasured acid, remembered as MUDPILES: Methanol, Uremia, DKA (and starvation/alcoholic ketoacidosis), Propylene glycol (lorazepam IV diluent), Iron and INH, Lactic acidosis (sepsis, shock, metformin, cyanide, carbon monoxide), Ethylene glycol, Salicylates. NORMAL ANION GAP metabolic acidosis (gap normal, hyperchloremia compensates for bicarbonate loss) is HARDASS: Hyperalimentation/parenteral nutrition, Addison disease, Renal tubular acidosis, Diarrhea (most common cause), Acetazolamide, Spironolactone, Saline infusion. Once high-gap is confirmed, calculate the delta-delta (change in gap divided by change in bicarbonate) to detect a coexisting normal-gap acidosis or metabolic alkalosis.