A rare side effect of mesalamine/mesalazine (canasa and asacol) is exactly what you're describing Renal Impairment. Has your gastroenterologist discontinued those medications just to see if your eGFR bounces back above 60? I'd call and ask as I believe he/she should have, I'm not a doctor so check with yours first. Liver damage can occur with mesalamines in some patients.
"Mesalazine should be withdrawn when renal impairment manifests in a patient with IBD; if this does not result in a fall in serum creatinine, then renal biopsy should be considered. A trial of high-dose steroid may be recommended in patients whose renal function does not respond to drug withdrawal. "
www.ncbi.nlm.nih.gov/pubmed/17243140"What does my eGFR mean?
A normal eGFR is 60 or more. If your eGFR is less than 60 for three months or more, your kidneys may not be working well. If you eGFR is below 15, you may need to start dialysis or have a kidney transplant.
The eGFR is a good test, but it’s not right for everyone. For example, this test may not be accurate if you are younger than 18, pregnant, very overweight or very muscular.
No matter what your eGFR is, ask your doctor when you should be tested again and what other tests you should have. Your doctor may want to look for other signs of kidney problems by doing more testing. These tests might include:
Urine (for protein called proteinuria or blood)
Blood (for other wastes)
Blood pressure
Blood sugar
Source :
www.kidneyfund.org/prevention/tests-for-kidney-health/