How are results from P1 NP and CTX affected by nonfasting blood draw?

I forgot to mention that I had had a shake with whey protein, milk and a banana earlier that day.

I believe the P1NP is not affected by food. The CTX should be early (11 is late) and fasting. I have read that it tends to be higher if not done early enough or fasting (see below). Same thing happened to me: I do not have a good baseline. If done consistently at 7-8 am fasting, this test is a lot more reliable according to my doc.

https://healthresearchfunding.org/c-telopeptide-blood-test-results-explained/#How%20Reliable%20Is%20This%20Blood%20Test?
Although the C-Telopeptide blood test can provide indicators that there is something going on that could be problematic, the test results are not considered to be definitive. The number of bone markers that are in the blood can change several times throughout the day. This is why fasting is encouraged because eating can cause an increase in specific markers.

Note also that biotin will affect the test:
https://www.labcorp.com/tests/500089/c-telopeptide-endocrine-sciences

You are to fast for these 2 tests and preferably in the early morning and each time you must repeat them, do so at the same time you had them before per Dr. Keith McCormick. Check out his videos on you tube.

P1NP does not have to be fasting but since you usually do them together, it will be! From online:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8252919/

PINP decreases 3.8% (+/− 0.9%) following intake of food. 15 The coefficient of cyclic variation of PINP is in the range of 3–5% which is very low. 16 Hence blood sampling can be performed at any time of the day regardless of fasting status, which is a major advantage of using sPINP as a bone marker. 17 Either serum or EDTA plasma sample may be used for PINP measurement and the results treated interchangeably as they are equivalent.

My doctor got upset when the nurse didn't tell me to fast before these two tests and is making me redo the blood test with fasting. So I'm definitely sure it makes a difference.

It makes a difference with CTX, not so much with P1NP, but we all do them together so basically both are done fasting. For CTX I was told to do it early (7-8am) and at a consistent time. @koleke glad your doc reordered.

One reason these tests might be seen as unreliable by some docs is apparently that they are done at different times and with and without fasting. McCormick says they are reliable if done right. My main endo doesn't use them because they "don't tell everything that is going on in bones." There are other tests as well, alkaline phosphatase, calcium, PTH etc. I am not sure myself. I get the tests done but I don't rely on them because they don't match my expectations or DEXA.

Dr McCormick says fasting, 7-8:30 am, no collagen or biotin for 48 hrs prior for the CTX. Also to take the repeat test at the same time for accurate comparison.

I have a problem with rigid times. Some people do not go to bed until 2 AM so 7 AM is not realistic. I could understand aiming for a period within 2 hours of awakening.

@normahorn it is hard to get lab appointments early too. I do mine around 9am consistently and hope for the best.