Is 24 hour urine collection an accurate measurement of blood volume?

[Zach Martin]

So back when I went to Mayo in 2019 I was diagnosed as hypovolemic based on a 24 hour urine volume. I also had elevated norepinephrine so I got the hyperPOTS label as well. As is custom, I was told to up my water/salt intake and exercise to increase my blood volume levels. However, when I water/salt loaded for about a year I actually experienced worse palpitations and gained a debilitating head pressure as newfound symptom. I wasn't monitoring my blood pressure at the time, but I imagine these symptoms were do to the increased salt elevating my bp which already runs a little high. 

What I'm trying to figure out now is if I actually do have low blood volume. I have read that the 24 hour urine collection isn't a very accurate measurement of blood volume--mine was measured at 1357 mL and my sodium was 198 mmol. I was drinking 2 liters of water per day at the time. From what I have read, my urine volume amount actually seems fairly average. The Doctor who I saw at Mayo also said I should cut back on my salt intake because that was apparently high. 

So my question is does anybody have any information on if 24 hour urine collections can be used to accurately measure blood volume? It sounds like the test is very dependent on how much fluid was consumed that day and is more of a test to measure how much salt one is eating rather than blood volume. 

[RecipeForDisaster]

I really doubt it - mine was over 3L and I definitely have low blood volume. I agree on the salt and fluid intake... mine were high that day as usual. I have trouble keeping my blood sodium high enough, which is so weird on 15gm salt daily.