A kidney panel looks like several independent measurements. Two of its most prominent rows are the same measurement twice.
Creatinine: measured
MedlinePlus describes where it comes from: "Creatinine is a normal waste product in your body. It's made when you use your muscles and some of the muscle tissue breaks down." And what the kidneys do with it: "Normally, your kidneys filter creatinine from your blood and remove it from your body in your urine."
So the concentration left in blood depends on two things — how much is being produced, and how much is being cleared. That both inputs matter is the reason the next row exists.
eGFR: estimated from creatinine
MedlinePlus is explicit: "Creatinine levels in blood are often used to calculate how fast your kidneys filter waste out of your blood. This is called an estimated glomerular filtration rate (eGFR). The calculation includes information about your age, weight, height, and sex."
Two consequences follow, and both surprise people reading their own reports.
eGFR is not an independent check on creatinine. It is computed from it. The two agreeing is arithmetic, not corroboration.
eGFR can change when creatinine does not. Because the calculation also takes characteristics such as age, a birthday can move the estimate while the measured value stays where it was. The formula itself is also periodically revised, and a laboratory adopting a new one produces a step change in the estimate with nothing having changed in you.
MedlinePlus notes the reason the estimate is printed at all: it "is a more accurate way to measure kidney health than creatinine levels in blood or urine alone."
Urea / BUN: a second, separate measurement
Blood urea nitrogen is a different waste product, measured rather than derived. Some reports print it as "urea", some as "BUN" — the two are related by a fixed factor, which is one more reason a value can look different between labs while describing the same sample.
MedlinePlus explains why it sits beside creatinine: "Creatinine blood levels measured as part of a CMP or a BMP may be compared with the level of BUN (blood urea nitrogen) that's measured in the same test. This can help find out the cause of a kidney problem."
The BUN/creatinine ratio is that comparison, printed as a row. It is a division of the two values above it, not a third measurement.
Reading the panel
Of the four rows people look at, two are measured — creatinine and urea — and two are computed from them. When a derived row moves, the first place to look is its inputs, which are on the same page.
And as everywhere, the interval that applies is the one printed on your own report, since MedlinePlus cautions against comparing to ranges "you might find elsewhere" — doubly so for an estimate whose formula the laboratory chooses.