Measuring creatinine and urea in the blood

This test is probably the most used test to monitor kidney function, but not a very sensitive one. Creatinine in the blood is usually measured when acute kidney damage or chronic kidney disease is suspected or monitored. Creatinine levels are also influenced by age, body weight, muscle mass, exercise, feeding, hydration status and intestinal absorption. Increase in creatinine is normally only seen when around 75% of the kidney function has been lost. 
Measuring the creatinine and urea levels in the blood is an important part of the diagnostic workup and follow up for diagnosis of suspected cases of JKD
Unfortunately this particular test can not be used as a screening tool to investigate the disease status of seemingly healthy animals.