5 CFR § 630.405 - Supporting evidence for the use of sick leave.
(a) An agency may grant sick leave only when the need for sick leave is supported by administratively acceptable evidence. An agency may consider an employee's self-certification as to the reason for his or her absence as administratively acceptable evidence, regardless of the duration of the absence. An agency may also require a medical certificate or other administratively acceptable evidence as to the reason for an absence for any of the purposes described in § 630.401(a) for an absence in excess of 3 workdays, or for a lesser period when the agency determines it is necessary.
(b) An employee must provide administratively acceptable evidence or medical certification for a request for sick leave no later than 15 calendar days after the date the agency requests such medical certification. If it is not practicable under the particular circumstances to provide the requested evidence or medical certification within 15 calendar days after the date requested by the agency despite the employee's diligent, good faith efforts, the employee must provide the evidence or medical certification within a reasonable period of time under the circumstances involved, but no later than 30 calendar days after the date the agency requests such documentation. An employee who does not provide the required evidence or medical certification within the specified time period is not entitled to sick leave.
(c) An agency may require an employee requesting sick leave to care for a family member under § 630.401(a)(3)(ii) to provide an additional written statement from the health care provider concerning the family member's need for psychological comfort and/or physical care. The statement must certify that -
(1) The family member requires psychological comfort and/or physical care;