Thread by @medrants

1/ #UncleBob shares #5goodminutes worth of lab nerdy thoughts from today's sadly interesting patient. These are thoughts that I often share on rounds. I feel that we should maximize what we can learn from the labs.
@CPSolvers @DxRxEdu @rabihmgeha @Anand_88_Patel

2/We heard that the patient had 5 days of diarrhea, but heard little more about that symptom. If the diarrhea had been profound then we could see a normal gap acidosis, therefore the labs suggest modest diarrhea.
125/4.5/83/26
Disappointing to me, we were not given BUN or creat

3/ Let's think about the Na of 125. That is low enough to cause altered mental status. Even patients who slowly lower their Na to that level often become confused.
The main differential for decreased sodium starts with volume contraction, euvolemia or edematous sate.

4/ Another nerdy, yet important point - by definition w/ a Na of 125 the patient is overhydrated. Volume contraction & dehydration are not synonyms. So the patient is overhydrated. Given the urine osms > 400 we can exclude primary polydipsia, beer potomania and "tea & toast"

5/ ADH is involved - is it appropriate or inappropriate
Appropriate ADH is like the post-op state - volume contraction, stress, pain, nausea, opiates. So we should be evaluating the patient's volume - hence I'm missing the BUN, creatinine and urinalysis.

6/ We might initially infer volume contraction with the elevated serum Hgb (17.9). While caring for this patient, I would want to see the Hgb decrease with volume.

7/ Towards the end we are given a clue that the patient actually has SIADH. If the patient had volume contraction, giving volume would increase the serum Na, but the Na decreased. This will only happen if the elevated urine osms persist. The explanation is a bit complex.

8/ Let's assume either LR or NS. They each have around 300 osms /liter. But if the urine osms are great than that, the patient will excrete the given osms in less urine than the IV fluids. The difference in volume is the equivalent of adding water to the intravascular space.

9/ If in fact the patient has SIADH (and small cell has SIADH in the 6-17% range), then what should we do? If money is no object, you can correct with Tolvaptan. When money matters you can try demeclocycline. With either, trying to normalize the Na is challenging.

10/ I hope that working through these labs and sharing my interpretations might help you with a future patient. Please ask questions in case I am guilty of the Curse of Knowledge and therefore did not explain things clearly enough.