Hello i hve just been told by my doctor that i will have to have another kidney blood test has the previous one was borderline what could be the probl?

Likely normal. : Could be a normal variant. With age our kidney function decreases slightly. It could be dehydration or due to supplement or medication use. Repeating it would be fine.

Many causes: If a kidney test was thought borderline abnormal, it may be that one of the labs that measures the kidney function is slightly elevated. Many times electrolytes are also measured during this test, which could also be mildly out of range. Sometimes, kidney function test abnormalities can be as straightforward as that one may have been dehydrated on the day of the test.

Creatinine: Sounds like your doctor is concerned about your kidney function.