[R-meta] It is possible to calculate the Hazzard Ratio and 95% CI on different reference groups from summary available data?

[Prof. Mario Petretta]

Dear mailing list members

I am doing a meta-analysis on the prognostic value of diastolic blood
pressure (BP). 
A study reported the unadjusted hazard ratio (from Cox regression) for three
categories: 

1)	BP under 90 - hazard ratio 1 (reference)
2)	BP 90 - 99 - hazard ratio 1.89 (95% CI 1.09, 3.33)
3)	BP over 100 - hazard ratio 3.03 (95% 1.67, 5.56)

It is possible to combine these hazard ratios and the 95% confidence
intervals for three categories in two categories, considering together the
reference category and the second category as a new reference vs the third
category? 

A more general question is: It is possible to convert hazard ratios and 95%
CI for 3 or more categories to a hazard ratio for 2 categories, changing the
reference category? 

Thanks for your attention.


___________________________________________
Mario Petretta, MD, FAHA
Associate Professor of Internal Medicine
Department of Translational Medical Sciences
Naples University "Federico II" - Italy