Many employers are frustrated with employees who frequently call in sick. And that frustration can lead to major headaches.
Tresa Baldas of The National Law Journal, writing in law.com, identifies several pending lawsuits filed by the EEOC where employers are apparently asking for too much information when employees use sick time. Of particular note is the 'intermittent leave' permitted under the Family and Medical Leave Act.
You've got to consult with your employment attorney or HR expert before making such a request.
In the meantime, I am an advocate of consolidating 'vacation' and 'sick' time into one broader category - paid time off (PTO). If such a program is in place, it doesn't matter why the employee is taking the time off. And once PTO is exhausted, the employee does not get paid for any additional time off - whether sick or vacation.