Our UDP team includes hospitalists, geneticists, radiologists, pathologists, and an access-coordinating specialist representing the majority of pediatric subspecialties. Each subspecialist brings to the evaluation experience and knowledge as both a pediatrician and a specialty physician, and they are invaluable in providing insights into the intricacies associated with specific symptoms or disease processes examined in the differential diagnosis.

The UDP inclusion criteria were as follows: undiagnosed despite evaluation by at least two specialists who assessed the patient for the objective findings. Patients who satisfied the inclusion criteria were referred to our UDP team from all around China. The UDP center registers the patient in the electronic health record and works with the family to obtain all medical records from every institution, clinic, and hospital that has cared for the patient, as well as copies of all actual imaging studies and slides of all tissue samples obtained by biopsy or tissue resection. The UDP liaison will assess if a patient satisfies the selection criteria after reading and summarizing the patient’s history. The UDP liaison is an attending pediatrician who is in charge of determining whether the patient matches the requirements, gathering medical information, and scheduling multidisciplinary expert meetings. Experts were invited by UDP center staff for consultation and debate. The 1.5–2 h team discussion always reveals fresh facets of the patient’s problems. Following the talk, patients will be scheduled for pertinent examinations and follow-up phone calls by UDP liaison and staff. This follow-up will occur every month after that. A unique file will be used to manage all patient data.

We employ various genetic testing methods to diagnose rare pediatric diseases, including karyotyping, fluorescence in situ hybridization (FISH), comparative genomic hybridization (CGH) and SNP arrays, single-gene sequencing, next-generation sequencing (NGS), targeted genomic and epigenetic analyses. Karyotyping is used to identify large-scale chromosomal anomalies, FISH detects specific DNA sequences, CGH and SNP arrays identify copy number variations, single-gene sequencing diagnoses specific diseases, NGS, including whole-genome sequencing and whole-exome sequencing, identifies rare and novel mutations, targeted genomics is used for multi-gene analysis in complex diseases, and epigenetic analysis is suitable for diagnosing disorders influenced by epigenetic factors. Each patient receives a unique genomic report, which is distributed to our clinician–scientist review panel and the patient’s physician. Sensitivity and specificity are analyzed for each genetic test by comparing test results with established diagnostic criteria or known genetic profiles. The treating physician ultimately has the last say over any new treatments recommended for the patient. Licensed genetic counselors are available to assist as needed. By incorporating detailed genetic and laboratory testing methods and sensitivity calculations, we aim to enhance the precision and reliability of diagnosing rare pediatric diseases (Figure 1).

In our hospital, we have 758 cases with comprehensive UDP project data from January 2017 to December 2021. Table 1 and Figure 2 show demographic data for these 758 patients. Males made up 436 of them (57.5%). Moreover, half of the patients were under the age of 5, with only 15 patients over the age of 14 (Figure 2A). Average time from symptom onset to admission to UDP was 6.0 months (95% CI 10.5–12.6), and 430 patients (56.7%) sought medical attention within 6 months of seeing symptoms (Figure 2B). These patients visited an average of 1.8 clinics during their diagnostic journey. Except for 69 individuals (90.9%), all had more than one presenting symptom in various organs: 460 (60.7%) had neurology problems, 151 (19.9%) had endocrinology problems, and 141 (18.6%) had immunology problems (Figures 2C,D). We discovered that the clinical phenotypic, growth and development problem was the most common, accounting for 249 instances (32.9%), followed by nervous system abnormalities (220 cases, 29.0%). UDP has a diagnosis rate of 61.3%. Genetic testing was performed on 469 of the 758 patients, for a genetic diagnosis rate of 15.8%.

We discovered that growth and development problems had the lowest confirmed rate of 113 (45.4%) and the longest mean duration before admission to the UDP, 14.7 ± 18.2 months, followed by the nervous system, 14.2 ± 14.5 months. The oncology and hematology system had the shortest duration, at 4.3 ± 7.9 months. The mean days to diagnosis following UDP admission for congenital multiple abnormality were the longest, 11.5 ± 12.9 days, and the shortest, 8.1 ± 4.5 days (Table 2). The metabolic diagnostic rate was the greatest, at 41.3%, while the rheumatic and immune system diagnosis rate was the highest, at 82.9%.

Table 3 displays the genetic mutations discovered in this UDP project. The most commonly found gene mutations in UDP was ABCD1, IGHMBP2, ATM, CSF1R, SLC25A1 and ACSF3 (Table 4). Adrenal leukodystrophy (ALD) is a rare genetic metabolic disease caused by a mutation in the ABCD1 gene on the X chromosome, which results in the inability to metabolize extremely long chain fatty acids (VLCFA) normally, resulting in the gradual loss of white matter in the nervous system and adrenal cortex degeneration. The majority of patients with cerebral type (cALD) develop total paralysis or possibly death within a short amount of time. SMARD1 (autosomal recessive spinal muscular atrophy with respiratory distress) is a severe form of spinal muscular atrophy that damages the diaphragm and can cause respiratory distress. A recessive mutation in the IGHMBP2 gene causes this condition. The ataxia telangiectasia mutated gene is a critical gene in DNA damage testing. It was discovered in patients with telangiectatic ataxia, and about 1% of the population had heterozygotes with ataxia-telangiectasia mutated (ATM) deficiency. CSF1R gene linked leukoencephalopathy is a rare autosomal dominant inherited leukoencephalopathy characterized by increasing neuropsychiatric and motor symptoms. Imaging results include diffuse white matter degeneration, corpus callosum thinning, and brain tissue calcification. Primary axonal degeneration and myelin sheath loss are pathological features. Children with congenital myasthenia syndrome (CMS) caused by the SLC25A1 gene have an early onset, are generally intellectually impaired or delayed, and have metabolic problems. The R247Q and V49M genotypes are repeatable. Mutations in the ACSF3 gene cause methylmalonuria, a rare autosomal recessive condition characterized by an increase in methylmalonic acid (MA) and malonic acid (MMA) levels in the urine. This gene encodes an enzyme that is involved in the production of fatty acids.

After UDP discussion, 501 of the 758 patients that joined the UDP received preliminary diagnosis. After genetic and other laboratory tests, 36 of them were misdiagnosed. The success rate for diagnostics was 61.3%. There were 257 cases that were not confirmed by UDP discussion, and 27 were confirmed by genetic and other laboratory tests. In the context of UDP method evaluation, positive gene test is taken as the golden standard. Sensitivity indicates how likely an UDP method is to diagnose an unknown disease when patient has a positive gene test. Specificity refers to the ability of an UDP method to rule out the presence of a disease in someone who has a negative gene test. Positive predictive value (PPV) is the number of correct positive results of an UDP method by the total number of positive results. Negative predictive value (NPV) is the number of correct negative results an UDP method gives divided by the total number of negative results. As a result, the UDP method has a sensitivity of 94.5%, a specificity of 86.4%, a PPV of 92.8%, and an NPV of 89.5% (Table 5).