length of hospital stay; number of days in hospital
Used in medical and administrative contexts to refer to the total number of days a patient stays in a hospital.
The average length of hospital stay for this surgery is about one week.
If the number of days spent in hospital is prolonged, medical costs increase.
入院期間 is a more general term for the period of hospitalization, while 在院日数 specifically refers to the count of days spent in the hospital, often used in statistical or billing contexts.
Compound of 在院 (ざいいん, 'being in hospital') and 日数 (にっすう, 'number of days'). The reading にっすう is a standard rendaku-influenced form of 日数.