From the Guidelines

Approximately 6% of pediatric IBD patients develop Primary Sclerosing Cholangitis (PSC), as reported in a cohort of 300 children with IBD 1.

A study published in the journal Gut in 2019 found that sclerosing cholangitis was reported in 6% of children with IBD, with a persistently raised γ-glutamyl transpeptidase being the most predictive marker 1.
The proportion of children with abnormal liver biochemistry who develop features suggestive of PSC is not well reported, but it is known that abnormal liver biochemistry in children with IBD is common, although most cases are not related to PSC 1.
Children with classic PSC have a disease pattern mirroring that of adults, with a poor response to treatment and a median survival before developing significant events or transplantation of 10–12 years 1.

Raised ALP is normal in growing children and adolescents and is unreliable for screening for PSC, making it essential to use other markers such as γ-glutamyl transpeptidase for diagnosis 1.
MRCP is recommended in children with AIH that responds poorly to medical treatment to screen for changes of sclerosing cholangitis 1.
Children with IBD should have regular liver function tests to monitor for the development of PSC, as early detection can help manage the condition more effectively.

The outcome of classic PSC in children is worse than for children with steroid-responsive AIH or ASC, resulting in shorter transplant-free survival 1.
Children with PSC often require liver transplantation at a young age and have a high rate of disease recurrence in the graft 1.

From the Research

The exact percentage of pediatric IBD patients who develop PSC is not directly stated in the provided studies.
However, a study from 2008 2 mentions that PSC affects up to 5% of patients with ulcerative colitis and up to 3.6% of patients with Crohn's disease.
Another study from 2001 3 found that 84% of pediatric patients with PSC also had IBD, but it does not provide the percentage of pediatric IBD patients who develop PSC.
A study from 2017 4 reports that 5 out of 21 patients with PSC had associated inflammatory bowel disease, but it does not provide a percentage of pediatric IBD patients who develop PSC.
A study from 2021 5 describes the characteristics and clinical course of children and young persons with IBD and sclerosing cholangitis, but it does not provide the percentage of pediatric IBD patients who develop PSC.
A study from 2016 6 examines the IBD phenotype in pediatric patients with PSC, but it does not provide the percentage of pediatric IBD patients who develop PSC.

The studies suggest a strong association between PSC and IBD, with the majority of patients with PSC also having IBD 4, 2, 3.
The most common type of IBD associated with PSC is ulcerative colitis, followed by unclassified IBD and Crohn's disease 5, 3.
The presence of PSC in patients with IBD may be associated with a higher risk of complications, such as colorectal malignancy and cholangiocarcinoma 2.