The best way to differentiate pre-renal from renal azotemia in this horse is to initiate IV fluids and recheck the kidney values (BUN, creatinine) in 12-24 hours.
This is most likely pre-renal azotemia based on clinical signs of dehydration and increased lactate with high-normal PCV and high TP. If pre-renal, kidney values should normalize within 12-24 hours of IV fluid therapy and rehydration. Left untreated, pre-renal can develop into renal azotemia.
Azotemia is divided into pre-renal, renal, and post-renal.
Pre-renal azotemia occurs when the glomerular filtration rate (GFR) decreases due to dehydration.
Renal azotemia occurs with intrinsic kidney disease (loss of > 70% functional nephrons).
Post-renal azotemia occurs with obstruction or rupture of the urinary tract and reabsorption of the BUN and creatinine.
Pre-renal azotemia can be additive with renal or post-renal azotemia (i.e., a horse with renal disease or an obstruction is also dehydrated).
Urine concentration is helpful in determining the source of azotemia (specific gravity > 1.020 if pre-renal) but it would be INappropriate to give furosemide to a dehydrated horse to induce urination. Dehydrated horses often do not pass urine thus obtainining a sample is impractical.
Fractional excretion of sodium can help distinguish between pre-renal and renal azotemia (should be < 1% in pre-renal), but wouldn't usually be the first step and is less practical than monitoring response to rehydration therapy (and again requires a urine sample).
A BUN-to-creatinine ratio of greater than 20:1 also points to pre-renal azotemia.
Check out this great review of azotemia, courtesy of Cornell University.
