I have performed a study evaluating the diagnostic performance of a new test (T) (binary outcome) for a disease (D). Logistic regression was performed with the disease state as a dependent variable, the performance of the new test (T), and a few previously published potential risk factors were used as the independent variable. It showed only the new test (T) is a significant variable.
I then used the independent sample t-test to evaluate the risk factors. It was significant for one of the risk factors (R1). I then repeated the logistic regression without the performance of the new test (T), R1 is now a significant variable.
Can I conclude R1 is a risk factor for D but because of the remarkable performance of the new test (T), it does not contribute in diagnosing D; or it is possible that I just don't have enough sample size to demonstrate the significance of R1 in the logistic regression model?