The number of referrals waiting to be treated topped 667,000 at the end of December 2023, covering an estimated 10 per cent of the Scottish population.
Without any increase in capacity, researchers calculate the waiting list will jump to nearly one million people by December 2026.
However, treating an additional 32,300 cases per year over the next three years could clear the backlog, experts suggest.
Covid impact
In the early stages of the pandemic, the NHS was forced to postpone elective, or non-urgent, treatments to focus resource on patients seriously unwell with Covid-19. This has led to an accumulation of people waiting to receive care.
Researchers from the Universities of Edinburgh, Oxford and Strathclyde conducted a country-wide analysis of Scotland's healthcare system over an 11-year period, from January 2013 to December 2023, to estimate the extent of disruption and long-term impact.
The waiting list rose from 285,000 in 2013 to 386,000 at the start of the pandemic in 2019 – a 35 per cent increase over six years. This steady rise suggests the service was already gradually declining before the pandemic, according to the study.
Covid-19 then intensified the decline, researchers say. By December 2023, there were 668,000 referrals waiting to receive non-urgent treatment – an increase of 73 per cent over four years. 78,000 of those had been waiting for more than a year, compared to just 3,000 in December 2019.
Longer waits
Backlogs were seen across all health boards in Scotland. By the end of 2023, more than half of inpatient and outpatient referrals in most regions were waiting longer than the Scottish Government's target of 12 weeks.
NHS Borders and NHS Fife were some of the worst-affected health boards for both inpatient and outpatient referrals, with more than double the number of ongoing cases in 2023 compared with 2019.
Researchers say that comparisons between regions should be interpreted with caution, as the figures do not account for differences in population demographics.
The medical specialties where most patients waited more than 12 weeks for both inpatient and outpatient appointments were ENT (Ear, Nose, and Throat), General Surgery, Gynaecology, Oral and Maxillofacial Surgery, and Urology.