Question of the Day Expires at Midnight, EST (New York)

On the patient board in The Pitt are numerous abbreviations that track important data. Among them are "WR" for "Waiting Room", "ESI" for "Emergency Severity Index", and "RFV" for "Reason for Visit". One such abbreviation on that board is "LOS". What does the "S" in "LOS" stand for?

Clues