Kidney Function Test Results

Both dehydration and protein intake can affect urea readings. I suspect you already know most of the below, but it may help (src: http://www.rixg.org/results/urea.html)

Urea is a small molecule produced from protein and put out by the kidneys, so blood levels rise in kidney failure. It is not such an accurate test for kidney function as creatinine, but is a useful test when used together with creatinine, because it is affected by:
How much protein you eat (it is low if you do not eat)
Whether you have taken in enough fluid (it rises if you are dehydrated)

Normal = 3.6-5.0 mmol/l (20-30 mg/dl).

Result
Comments
below 3.5
Low. Common explanations are low protein intake from diet, or liver disease. A low urea is normal in pregnancy.
3.6-5.0
Normal
7.0+
High. Suggests kidney failure, dehydration, high protein intake, or bleeding. Levels over 8-10 are unlikely to be due to diet alone.
25+
Very high - suggests serious illness or renal failure
Urea has given its name to 'uraemic', which means the symptoms and signs of kidney failure.

lol @Element137 Not sure about "expert" status, but I've definitely had a baptism of fire on CKD, transplants and dialysis over the last couple of years, so have done plenty of research as a result. When looking at Kidney function, I believe the eGFR is the result you should be more interested in, as this shows your estimated kidney function, and takes creatinine into account. Don't be disappointed if you don't get "100" on you eGFR, as a) it is an estimate (and just like investments, your eGFR "may go down, as well as up" from test to test) and b) your kidneys start to degrade form the day you are born. The trick is to simply prolong them, and in case you are interested, the simplest ways to do this is maintaining low/steady BGs and low BP.

My wife has stage 3/4 CKD and is suspected of having had damage due to long term medication for frozen shoulder, colitis etc. Her Creatanine is going up. Not many people know that ibuprofen can damage the kidneys if taken for too long; same for several other drugs. As ever don't take more drugs than you really need and for longer than needed.

Dehydration due to too little salt is one possible reason, don't expect your GP to understand that we need a little more salt if low carb.

Did you fast for this test? I only ask because if you fasted overnight and then didn't drink plenty of water in the morning you possibly were dehydrated without realising.

this may help if you want some info before speaking to your doctor (src: http://www.rixg.org/results/egfr.html)

eGFR is a test of kidney function. It uses the result of your Creatinine test with your age and sex to estimate an approximate percentage kidney function. eGFR is only an approximate number, but gives a better idea of kidney function than Creatinine alone. If you are of black African origin, you should add 20% to the figure (this correction is unlikely to have been made in the lab). Normal is over 90 ml/min/1.73m2. 'Normal' eGFR is the same for all ages and for men and women. Calculating eGFR doesn't make sense in patients on dialysis. If you are on dialysis and an eGFR is shown, ignore it.

Different ways of calculating eGFR. There are now two different methods to calculate eGFR. The MDRD method is still more common, but the CKD-EPI method is growing. The two methods give similar results when kidney function is poor, but CKD-EPI results tend to be higher, and more accurate, in people with quite good kidney function (especially over 60). It isn't easy to tell what method has been used, but different methods could explain changes in eGFR when Creatinine hasn't changed much.

Result/Comments
over 90 Normal

60-90 This may be slightly reduced, but because eGFR is only an estimate of kidney function, many people with eGFR 60-90 are normal. An eGFR of 60-90 should only be used as evidence of kidney disease if there are other things wrong - such as protein or blood on urine tests, or other tests showing kidney disease - or if it is a big change from a previous eGFR.
30-60 Moderately severe kidney damage. This is known as Stage 3 CKD
15-30 Severe kidney damage, Stage 4 CKD. Anaemia and bone disease become common problems as kidney function drops below eGFR 25.
below 15 This is known as Stage 5 CKD. An average figure for commencing dialysis in the UK is 8 to 10, but this varies a lot - lower in some people, higher in others.

eGFR is only reliable in patients who are reasonably well, and not in hospital. Even then it may be quite inaccurate in some people - usually people with much more or much less muscle than usual. So it will be misleading in people with paralysis, or who have lost a leg, or who have lost a lot of weight (muscle).

More info about eGFR from the National Kidney Federation

More info about the stages of kidney disease from EdREN