Martin McGuinness’ surprise withdrawal from a ministerial visit to China was due to medical issues, his office has said.
The Sinn Fein veteran pulled out last week citing unforeseen personal circumstances.
The Executive Office has now said the late withdrawal was to enable doctors to conduct a medical assessment.
Mr McGuinness has been at work at Stormont while Mrs Foster has been in China.
An Executive Office spokesman said: “The Deputy First Minister withdrew from a planned visit to China this week on medical advice and to enable doctors to conduct a medical assessment.
“Martin McGuinness is fulfilling his responsibilities as deputy first minister working with his departmental political advisers.
“There has been no disruption to the work of the Executive Office during this period.”
Sinn Fein president Gerry Adams wished his long-time friend and his wife Bernie well.
“I’m sure everybody who values his work will send best wishes to Martin, Bernie and their family,” he said.
“I met with Martin as normal at Stormont on Monday. I have been in regular contact with him since and he is totally committed to fulfilling his duties.”