@BellaDonna1959 you'll find from the voices here no consistency in the answers to your questions. The CA scores are not hard wired. The test itself is at best only an marker but has no reliability for accuracy. As an example, when I was first diagnosed in 2017 my CA was in the 900 range, with one tumor the size of a large grapefruit and several smaller ones on my other ovary and scattered with in my peritoneum. But the CA for my last recurrence 5 months ago was 7500, yet the two tumors that they monitor are only millimeters in size. What they look to to estimate the efficacy of your chemo is the trend, not the score. If it's consistently going down, no matter how much, it's an indicator the chemo is working. Definitively, it's the scans that really provide a more accurate picture. Trying to rely on a CA score after only one treatment is dangerous for your emotional health. The efficacy of your treatment only increases over time. That's why you're generally told how many cycles to expect...usually six in primary treatment, either all following surgery or split pre and post surgery but everyone is different so you can't rely on that other than as a general benchmark.
As for platinum resistance, it's generally assessed based on the success of your first line treatment. If your CA scores trend up, and your scans show progression while on treatment, or within six months of treatment then there is cause to consider you may be platinum resistant and another line of treatment may be more appropriate. But it's way too early, and especially with your first CA showing a decline, to even think about resistance or any change to the treatment you're receiving.
I'm not sure what 'sites' you're using to research your treatment but please be careful There are many out there that are unreliable in the accuracy of the information they provide. And definitely there is not one answer that reflects what expectations should be for everyone. I am so sorry your nurse responded to your questions the way she did. Your CA score should have been very hopeful information and if the chemo is working you should be seeing ongoing trending in the right direction. . That said, you may also find blips from time to time. The CA125 is not just reactive to cancerous tumors, it will react to other abdominal inflammatory conditions. Don't panic, stay with your treatment and assess over time and the bigger picture. The treatment you are receiving is a practice standard for those of us with High Grade Serous OVC and generally very effective.