Blood test values pre or post dialysis?

When is the correct time to draw blood for tests? Before or after dialysis?

We have about 5 liters of blood in our body. Now I usually put on about 3 kgs of fluid between treatments. Most blood tests have results that are volume specific. That is, they are reported as something per milliliter.

Now if the blood is drawn before dialysis, the blood is usually diluted with water because I have extra fluid in my body that has not been removed. So, the test value that is reported will be less than actual because if say, 4 units of a substance exist per ml of my blood, the blood drawn will contain less than that per ml because some of it is actually not my blood but fluid.

I realized this because a lab once inadvertently did my Liver Function Tests on both my pre Dialysis and post Dialysis samples (I had given pre and post samples to actually check the change of Serum Potassium and Serum Phosphorus during dialysis). The values were all different! And liver function can definitely not change so drastically over a 8 hour period!

Can someone throw some light on this?


Karthik said…
Would the Pre dialysis sodium level would be low because of dilution?
Kamal D Shah said…
I think it would be lower, yes.