A proposal in the Senate to limit tax deductions for medical costs would fall hardest on middle-income taxpayers who are uninsured and who come up against expensive health problems. Currently, a taxpayer may claim a deduction for medical or dental expenses only to the extent those costs exceed 7.5% of the person's adjusted gross income. Senate negotiators are discussing raising that threshold to 10%, in effect denying deductions for many taxpayers who could claim them today.