Using a long-term follow-up protocol, we retrospectively evaluated a cohort of patients followed at our center between 2000 and 2023 for a congenital SFK caused by UKA or UMCDK. Some of these patients (46 out of 160) had been enrolled in our earlier prospective multicenter study entitled “Renal Parenchymal Thickness in Children with Solitary Functioning Kidney” [9]. The current cohort of patients included children with UKA or UMCDK identified by prenatal or postnatal US screening. In the Czech Republic, prenatal US screening is performed as part of recommended gynecological procedures in all pregnant women and is covered by insurance. With regard to renal defects, screening at 20–22 and then at 30–32 weeks’ gestation is essential. In addition to regular prenatal US screening, US screening for renal defects is performed in all newborns aged 3 to 4 days in our center.

In infancy, SFK was confirmed by DMSA static renal scintigraphy and was defined as 100% unilateral function. In patients with significant urinary tract dilatation, MAG3 diuretic renal scintigraphy, as an alternative to DMSA scintigraphy, was performed to confirm SFK.

The exclusion criterion was having diseases other than CAKUT that could affect renal function in the neonatal period, such as sepsis or perinatal asphyxia. The cohort size was determined by a statistician based on literature data and a pilot analysis of some of our patient cohort.

The indicators of kidney damage were reduced GFR and signs of hyperfiltration injury, that is, hypertension and proteinuria. The category of combined kidney damage comprised children with any of the above indicators, that is, reduced GFR or hypertension or proteinuria.

The GFR was calculated using two formulas based on the child’s serum creatinine level, height, age, and sex, namely the Schwartz formula [10, 11] and CKiDU25 [12]. In children aged 2 years or older, GFR < 90 mL/min/1.73 m2 was considered reduced and GFR < 60 mL/min/1.73 m2 was considered moderately reduced. The reduced GFR category included children whose GFR was found to be reduced using both formulas. According to the KDIGO guidelines, long-term GFR < 90 mL/min/1.73 m2 is referred to as stage 2 chronic kidney disease (CKD 2) and GFR 60–89 mL/min/1.73 m2 as stage 3 chronic kidney disease (CKD 3). In children younger than 2 years of age, GFR was assessed according to the National Kidney Foundation recommendations [11].

Casual oscillometric blood pressure (BP) measurements were obtained in all children during regular outpatient visits. In patients under 3 years of age, only the oscillometric method with a proper cuff was used (noninvasive BP measurements using a vital sign monitor). In children older than 3 years of age, sitting oscillometric BP measurements were primarily taken following 3 to 5 min of resting. In case of high BP, measurements were repeated twice, taken in both arms, and the second and third readings were averaged. If the resulting value was still high, measurements were repeated in a similar manner using the auscultatory method. For children over 5 years of age with high casual BP, ambulatory BP monitoring (ABPM) was added.

Hypertension was defined as BP ≥ 95th percentile for the sex, age, and height of the child [13] at casual measurement and BP ≥ 95th percentile for the sex and height of the child at ABPM [14]. Children with hypertension at casual BP measurement had other secondary causes of hypertension excluded.

In addition, some patients older than 5 years (65 out of 160) underwent 24-h ABPM as a screening method for detecting masked or nocturnal hypertension.

Proteinuria was assessed by calculating the urine protein–creatinine ratio (uPCR) and/or urine albumin–creatinine ratio (uACR) in early morning urine samples. Proteinuria was defined as uPCR > 20 mg/mmol and/or uACR > 3 mg/mmol in children aged 2 years or older and as uPCR > 50 mg/mmol and/or uACR > 10 mg/mmol in children younger than 2 years [15].

Children with hypertension and repeated proteinuria or albuminuria were treated with ACE inhibitors or angiotensin receptor blockers. These children were categorized as having hypertension or proteinuria, even though their BP or urine proteins were controlled with medication.

Following on from previous studies on congenital SFK, we investigated the following risk factors: sex, SFK side, immaturity (preterm birth before 37 weeks’ gestation), low birth weight (< 2500 g), GFR at initial examination, SFK length, CAKUT in SFK, urinary tract infection (UTI), body mass index (BMI) at final examination, hypertension, proteinuria/albuminuria, and urinary beta-2 microglobulin (U-B2M).

SFK length was measured by US and classified into percentile intervals according to the mean kidney length by age, as described by Akhavan et al. [16].

CAKUT in the solitary kidney was diagnosed by US and, in selected patients, by MAG3 diuretic renal scintigraphy to detect pelviureteric junction obstruction (PUJO) and by micturating cystourethrography (MCUG) to detect VUR. The severe CAKUT category included obstructive uropathy of SFK (PUJO, primary obstructive megaureter) requiring surgery or nonobstructive hydronephrosis with an anterior–posterior diameter ≥ 15 mm or megaureter ˃ 7 mm. Also, grade 4–5 VUR and refluxing megaureter were classified as severe CAKUT.

UTI was defined as the presence of pyuria and significant bacteriuria.

BMI was assessed using national reference charts for sex and age [17] and categorized as normal, overweight (> 85th percentile and ≤ 97th percentile), and obese (> 97th percentile).

The indicator of tubular injury was U-B2M, with levels above 0.202 mg/L being considered elevated by our laboratory.

The follow-up protocol was similar for all patients, but evolved over the years. Children who were diagnosed with UKA or UMCDK by prenatal or postnatal US screening were followed in our outpatient nephrology unit. In the case of normal postnatal US findings in the functional kidney, the first appointment was scheduled at approximately 3 months of age, which included a follow-up US examination and DMSA static renal scintigraphy to confirm the function of only one kidney. At the same time, GFR was measured (using serum creatinine levels) and urine tests were performed, including uPCR and/or uACR. Parents were informed about the symptoms of UTI. In selected children with dilatation of the SFK pelvicalyceal system, antibiotic prophylaxis for UTI was administered. With normal US findings in SFK, the follow-up US scan, urine tests, and BP measurements were scheduled at 1 year of age, then annually until 5 years of age, and every 2 years thereafter. Initially, over the first 7 years of following the cohort, MCUG was indicated in all children with SFK. Later, the follow-up protocol was changed so that MCUG was only performed in selected patients—those with SFK ureteral dilatation on US, SFK length below the 5th percentile, or UTI.

Initially, proteinuria was monitored, later albuminuria was added, and in the last 5 years, U-B2M was also measured. Preventive ABPM was performed only in the last 10 years. In the case of any pathological finding (SFK dilatation on US, proteinuria, reduced GFR, hypertension, UTI), check-ups were more frequent. Patients with obstructive SFK defects underwent early surgery by consensus of a nephrologist and a urologist. Surgery was also indicated for high-grade VUR and febrile UTI occurring despite antibiotic prophylaxis.

Patients were followed at our center until 18–19 years of age. After that, further follow-up was recommended, either by a general practitioner or, if kidney damage was suspected, by a specialist, usually an adult nephrologist. The detailed follow-up protocol is shown in Figure S1 in the Supplementary Material.

Two independent samples were compared using Fisher’s exact test (qualitative data) and Mann–Whitney U-test (numerical and ordinal data).

Risk factors for reduced GFR and hypertension were assessed with odds ratio (OR) and 95% confidence interval (95% CI). The former was calculated by logistic regression. In addition, multivariate logistic regression was used to assess risk factors for reduced GFR and hypertension. In both cases, the model included the following variables: sex, BMI, immaturity, low birth weight, recurrent UTI, hypertension, proteinuria, elevated U-B2M, SFK side, SFK length at 1 year of age, GFR at initial examination, and severe CAKUT. The model was developed using the forward stepwise method (likelihood ratio).

All tests were performed at a significance level of 0.05. Statistical analyses were performed with IBM SPSS Statistics 23.0 (Armonk, NY: IBM Corp.).