29 y/o F with history of microvillous inclusion disease complicated by chronic diarrhea, on home total parenteral nutrition (TPN) and endocarditis complicated by immune-complex glomerulonephritis presented to ED with 2 days of palpitations, dyspnea and fatigue in the context of 2 day history of increased stool output and TPN non-adherence.
She was found to have non-gap hyperchloremic metabolic acidosis with HCO3 of 2.7, Cl 117, AG 13, pH 7.09, pCO2 17, cre 0.7, BUN 15, UA with 1+ blood and 1+ protein.
She was started on bicarbonate drip in the ED and TPN was restarted. HCO3 normalized to 27 and pH to 7.4 within 10 hours. On arrival to the floor urine electrolytes were tested and anion gap (UAG) was positive. As CO2 corrected following bicarb gtt and resumption of TPN, the most likely causes would be GI, however renal tubular acidosis still remains in the differential as UAG is difficult to interpret after HCO3 infusion.
What would be your approach to the case?