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From the ABA 1990 ITE (Book A, question 76)
Postoperatively a 70-kg patient has a serum sodium value of 130 mEq/L. To increase this value to 140 mEq/L would require the administration of how much sodium in mEq?
(A) 140
(B) 250
(C) 420
(D) 600
(E) 740
The ABA's key says the answer is A.
My understanding is that a sodium deficit is calculated based on total body water (60% of mass in adult males), so
A couple of sources agree with using TBW:
Longnecker actually includes an example very similar to this questions:
Is the key wrong or is there something else here I'm not getting - ie, is there a difference between the total body Na deficit and what you need to administer to raise the plasma concentration?
Postoperatively a 70-kg patient has a serum sodium value of 130 mEq/L. To increase this value to 140 mEq/L would require the administration of how much sodium in mEq?
(A) 140
(B) 250
(C) 420
(D) 600
(E) 740
The ABA's key says the answer is A.
My understanding is that a sodium deficit is calculated based on total body water (60% of mass in adult males), so
- 70 kg x .6 = 42 kg (L) of water
- 42 L x (140 - 130) mEq/L = 420 mEq
A couple of sources agree with using TBW:
- http://www.fpnotebook.com/Renal/Lab/TtlBdySdmDfct.htm
- p 637 of Longnecker
Longnecker actually includes an example very similar to this questions:
Longnecker said:If the patient's weight is 70 kg, and the serum sodium is 120 mEq/L, then the desired change is 10 mEq/L.
Total body deficit (TD) of sodium is the sodium deficit x TBW:
NaD x (weight in kg x 0.6) = TD
The formula is applied as follows:
10 x (70 x 0.6) = 420 mEq
Is the key wrong or is there something else here I'm not getting - ie, is there a difference between the total body Na deficit and what you need to administer to raise the plasma concentration?