High Composite, Low Writing- applying to Columbia

I am an international student doing the IB Diploma at an international school in China. I will be applying Early Decision to Columbia University this year. I took the ACT with writing in April and June and respectively got 34 Composite/25 writing and 35 Composite/19 writing. Although Columbia does not require ACT's writing test, I am worried that it will reflect poorly on how ready I am for college writing and also raise issues about the authenticity of my application essays. Should I retake the test?

I don't think you should bother retaking the test. Your composite scores are excellent, and schools that don't require the Writing section typically won't consider Writing scores at all in the application process even if you submit them. There have also been some issues in the past year with scores for the ACT Writing section being unreliable, so even if Columbia did look at your Writing scores, they wouldn't necessarily consider them a valid assessment of your abilities.