MOFFITT AM REPORT PEARLS 8/9/16: Non-Gap Metabolic Acidosis and Base Deficit!

Thanks to Dan Selvig for presenting a combo GI and renal case this morning that managed to loop in HIV, diarrhea, urine anion gap, and so much more! 🙂


Top pearls from today’s report:

  • Use urine anion gap to work up a non-gap metabolic acidosis.
  • Base deficit/excess are mainly used for fluid resuscitation in the trauma/surgical literature.



HIV + diarrhea: See several prior blog posts on this and related topics pertinent to our discussion this morning from all 3 sites! J







Urine Anion Gap is the next step in evaluation of a patient with a non-gap metabolic acidosis where the diagnosis is clinically unclear.

Quick ddx for non-gap metabolic acidosis:

  • GI loss of bicarb (diarrhea)
  • Renal acid/base dysfunction (RTA)
  • Normal saline (dilutes bicarb)
  • Carbonic anhydrase inhibitors (renal loss of bicarb)

Goal is to determine whether acidosis is due to a renal or GI origin (also consider iatrogenic saline-induced acidosis).

UAG = U(Na) + U(K) – U(Cl)

UAG is surrogate for urine NH4+ (unmeasured cation).

Negative UAG implies excess urine NH4+ (unmeasured cation), which means kidney is appropriately excreting acid (in the form of NH4+) to compensate for acidosis likely of GI or other non-renal origin (e.g. diarrhea or iatrogenic normal saline).

Positive UAG implies inadequate NH4 excretion (distal type 1 RTA).

In cases where UAG is inaccurate (e.g. unmeasured anions such as ketoacids), use urine osmolar gap (a discussion for another time!).

Base deficit/excess: Used as a marker for volume resuscitation in trauma or burn patients. Definition is “the amount of base, in millimoles, required to titrate 1L of whole arterial blood to pH 7.40 (O2 sat = 100%, T = 37.0, pCO2 = 40)”

  • Calculates the amount of blood buffer (hemoglobin + plasma bicarbonate) at a given time.
  • Calculated from ABG using pH and bicarb values (formula too complex to memorize, based on Henderson-Hasselbach equation)

Base deficit implies metabolic acidosis.

Base excess implies metabolic alkalosis.

Normal base deficit = -2 to +2

Mild base deficit = -5 to -3

Severe base deficit: < -10

There is evidence that base deficit is a reliable physiologic marker of tissue perfusion in hypovolemic shock and may indicate resuscitation requirements in critically ill patients. Most of this comes from trauma literature.

It’s not clear on literature review how to apply this to medicine patients. Overall, I would stick with our usual markers of perfusion and use this as just another parameter in our overall volume and acid/base assessment.



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