Hi Dr. Topf,
(I don’t have Twitter) I wanted to ask you about question 6 on the week 2 quiz:
“An unresponsive woman is brought to the emergency room. She has a history of a suicide attempt a few years earlier. The lab tests are: Serum Na 140 mmol/L Serum K 4.0 mmol/L Serum Cl 100 mmol/L Serum HCO3 14 mmol/L, BUN 17 mg/dl, creatinine 0.7 mg/dL, serum osmolality 323 mOsm/Kg, Blood glucose 72 mg/dl, Blood gases: pH 7.28 pCO2 27 mmHg. What would you expect the urine pH to be in this patient?”
Why is it that we would expect the urine pH to be acidic? Since blood pH is 7.28, I would imagine that urinating out HCO3- (explaining the low serum HCO3) would have caused the acidic blood pH, thus making urine pH basic?
Thanks for your help,
“An unresponsive woman is brought to the emergency room. She has a history of a suicide attempt a few years earlier.
This is the “tell” of the stem. Acid base + suicide = ethylene glycol toxicity
The lab tests are: Serum Na 140 mmol/L Serum K 4.0 mmol/L Serum Cl 100 mmol/L Serum HCO3 14 mmol/L, BUN 17 mg/dl, creatinine 0.7 mg/dL , Blood glucose 72 mg/dl,
They don’t tell you the anion gap. Calculate it.
High anion gap.
serum osmolality 323 mOsm/Kg
More of the tell. They won’t tell you the osmolality unless they want you to calculate the osmolar gap (or it is a hyponatremia question)
Osmolar gap = 323 – (280 + 4 + 6 + 0)
Osmolar gap = 323 – 290
Osmolar gap is a massive 33 (Upper limit of normal is 10, over 20 starts to gain a lot specificity for toxic alcohol)
This confirms our earlier suspicions of ethylene glycol toxicity
Blood gases: pH 7.28 pCO2 27 mmHg. What would you expect the urine pH to be in this patient?”
The ABG confirms the metabolic acidosis.
Let’s do Winters formula (not really needed for this question, but you know…practice)
1.5 x 14 =21 + 8 =29, measured CO2 is within ±2 of predicted so an appropriately compensated metabolic acidosis.
Why is it that we would expect the urine pH to be acidic? Since blood pH is 7.28, I would imagine that urinating out HCO3- (explaining the low serum HCO3) would have caused the acidic blood pH, thus making urine pH basic?
So the bicarbonaturia you are talking about would happen if the cause of the metabolic acidosis is renal loss of bicarbonate (what we call renal tubular acidosis).
The urine would also be acidic if the patient had a non-anion gap metabolic acidosis from diarrhea.
Hope this helps