Cystinosis is a rare congenital, inherited metabolic disorder that results in the storage of cystine in the cells of many organs of the body. In the infantile nephropathic form of the disease, only the kidney is initially affected by a loss of function, which progresses if untreated and ends in terminal renal failure by early school age. With the prolonged survival of patients due to medication and renal replacement therapy, further loss of function may occur during the course of the disease, especially in the eyes, muscles, endocrine organs and central nervous system.

The quality of life of children with cystinosis is an under-researched topic. The results of the studies available so far show that the young patients and their families report a reduced quality of life and sometimes behavioral problems.

To date, there are no disease specific patient reported outcome measures (PROMs) to measure the quality of life of patients with cystinosis. The aim of the study is to develop a PROM for this target group in several languages (German, English, Spanish and French) from different countries (Germany, United States, Spain, France). The PROM will focus on quality of life and will be developed for children, adolescents, and young adults including parent-report of parents with children aged 0 to 26 years.

First, a literature review was conducted to identify relevant quality of life topics for the focus interviews. The development of the cystinosis-specific PROM will include three phases with patient recruitment: (1) focus interviews, (2) pilot-test and cognitive debriefing, and (3) field and re-test.

1. Focus interviews with up to 25 parents (of young patients aged 0 to 26) and 15 young patients (ages 8-26) per country (Germany, France, Spain and USA) will discuss aspects relevant to the young patient's quality of life. A pilot instrument version will be developed based on the interviews and then translated into the other project languages according to ISPOR guidelines.
2. The pilot instrument (translated) will be administered to patients and parents along with a cognitive debriefing to assess the comprehensibility, interpretation, and cultural relevance of the items. Up to 200 parents (25-50 per country) and 120 young patients (15-30 per country) will complete the pilot instrument and cognitive debriefing.
3. At least 300 parents (75 per country) and 180 young patients (45 per country) will complete the refined questionnaire as part of a field and re-test.

The questionnaire will be filled out again after two weeks by at least 20% of the patients and parents to assess the test-retest reliability.

The final product will be a psychometrically validated, easy to use, and conceptually appropriate quality of life instrument available in German, English, Spanish, and French for use in research and patient care.

In all study phases, patient recruitment follows these inclusion criteria:

Patients will be asked to participate in the study if they meet the following inclusion criteria:

• Children, adolescents, and young adults aged 8-26 years (and at least one of their parents) and further parents only of children aged 0-7
• patients have a confirmed diagnosis of cystinosis
• patients have a sufficient knowledge of the German/ English/ French or Spanish language to participate in focus interviews and complete questionnaires
• the informed consent of legal guardian and assent from the patient (if older than eight years) was given

Exclusion criteria:

• severe cognitive impairment
• other severe illnesses that strongly determine everyday life

The participants will be recruited by the Interdisciplinary Cystinosis Clinic in Rosenheim 'RoMed' clinics, the German Cystinosis Foundation, the Cystinosis Research Network, the Asociación Cistinosis Espana, the Necker-Enfants Malades Hospital, the Hospices Civils de Lyon and the VML - Vaincre les Maladies Lysosomales