BUN/Cr, FENa

Question 1

What does BUN/Cr stand for?

Answer 1

Blood urea nitrogen to creatinine ratio

BUN/Cr is a ratio used to assess kidney function.

Question 2

What does FENa represent?

Answer 2

Fractional excretion of sodium

FENa indicates the amount of sodium in the urine.

Question 3

What is the ‘normal’ range for BUN/Cr and FENa?

Answer 3

There’s no ‘normal’ range for these values

The interpretation of these values depends on the context of kidney problems.

Question 4

What do BUN/Cr >20 and FENa <1% indicate?

Answer 4

Pre-renal azotemia

These values suggest that the kidney problem is related to pre-renal causes.

Question 5

What do BUN/Cr <20 and FENa >1% indicate?

Answer 5

Not pre-renal

These values suggest that the kidney issue is not due to pre-renal factors.

Question 6

What does the notion of intra- vs post-renal ranges imply?

Answer 6

It is considered nonsense for USMLE

This distinction is not relevant for understanding kidney function in the context of USMLE.

Question 7

What is the reliability of BUN/Cr values in NBME questions?

Answer 7

Only ~9/10 times the BUN/Cr will be as expected

There are exceptions where BUN/Cr <20 may indicate pre-renal azotemia.

Question 8

In case of an exception in BUN/Cr values, how should the diagnosis be approached?

Answer 8

The vignette will be overwhelmingly obvious for the diagnosis

Even if values appear contradictory, the clinical context will clarify the diagnosis.