The current research is part of a 10-year retrospective cohort study of patients registered with the Pediatric Nephrology Department at Medical University of Bialystok, Poland. Demographic, clinical, and laboratory data were obtained. Hematological indices　 (red and white blood cell count, hemoglobin, hematocrit, platelet count, and mean platelet volume) serum creatinine, urea, uric acid, total cholesterol, triglycerides, urinalysis, and glomerular filtration rate (GFR) were assessed.

The inclusion criteria were: 11-　to　21-year-old　adolescents with serum uric acid >5.5 mg/dL who were referred to our department in the primary care office because of elevated casual blood pressure. The　exclusion criteria were heart failure, diabetes mellitus, renal or hepatic dysfunction, hematological disease, systemic inflammatory conditions, autoimmune diseases, secondary hypertension, and　treatment with medications that affect uric acid levels and blood pressure values.

The Bioethics Committee of the Medical University of Bialystok approved the protocol. Bodyweight and height were measured using a balance beam scale and a wall-mounted stadiometer, respectively. Body mass index (BMI) was calculated using standard formula. Age- and gender-specific reference values for BMI were generated by the LMS method [7]. The LMS values were taken from the study by Kulaga, et al. [8]. We defined metabolic syndrome according to the criteria established by Cook, et al. [9]. The diagnosis of hypertension was based on recommendations of European Society of Hypertension [10], and hyperuricemia was defined as serum uric acid level >5.5 mg/dL. Overweight and obesity were defined using criteria developed by Cole, et al. [10].

Blood samples were collected in the morning, after overnight fasting. The venous blood samples were combined with dipotassium EDTA and tested within 30 minutes of collection. Complete blood counts were measured using an automated blood counter. Serum creatinine was determined by updated Jaffe reaction and uric acid was assessed using the colorimetric method. Serum cholesterol, HDL-cholesterol and triglycerides were determined by the enzymatic method using Hitachi 912 (La Roche Japan). Serum glucose was measured with the Integra 800 analyzer. GFR was assessed by updated Schwartz formula [11].

We examined 607 adolescents (474 boys) during the study period; 187 (31%) had metabolic syndrome. In the　adolescents with hyperuricemia but without meta-bolic syndrome (420), primary hypertension was found in 48% and obesity in 19.7% patients. There was a　statistically significant positive correlation between serum uric acid level and BMI-Z-score (P<0.001), white blood　cell　count　(P<0.001) and red blood cell count (P<0.001), hemoglobin (P<0.001), and　hematocrit (P<0.001).

The study showed statistically significant correlations between white blood cell count and lipid profile in hyperuricemic adolescents. White blood cell count correlated positively with total cholesterol (r =0.14, P<0.001) and triglycerides (P=0.032), and negatively with high-density lipoproteins (P=0.045). Additionally, a　positive relationship was found between BMI Z-score and platelet count (P=0.002) and between　BMI Z-score　and　red blood cell count (P=0.005), and a　negative relationship between BMI Z-score and mean platelet volume (P=0.038) (Fig. 1).

Fig. 1 Comparison of hematological parameters between adolescents with (MetS (+)) and without (MetS (-)) metabolic syndrome.

The results showed significantly higher hemoglobin, hematocrit, and red blood cell count (P<0.001, P<0.001, and P=0.032, respectively)　in patients with metabolic syndrome　than　in those without this diagnosis.

The results of this retrospective study showed that in adolescents with hyperuricemia, serum uric acid levels correlated with white and red blood cell count, hematocrit, and hemoglobin. We found significantly higher levels of hemoglobin, hematocrit, and red blood cell count in adolescents with metabolic syndrome than in the　group without it.

Our findings are in general agreement with the earlier reports [3,12,13] in children and adolescents with obesity, hypertension, and/or metabolic syndrome. Increased insulin resistance and other components of the　metabolic syndrome were earlier shown to be associated with increased white and red blood cell count, hemoglobin, and hematocrit, and as　a consequence, increased blood viscosity [14]. Our finding of no correlation between serum uric acid and platelet count and mean platelet volume was contrary to other reports [15] Although there is some evidence linking increased white and red blood cell count, hemoglobin, and hematocrit with hyperuricemia, role of confounding variables like chronic inflammation and obesity-associated obstructive sleep apnea cannot be ruled out.

This preliminary study suggests that increased hematocrit, hemoglobin, and white and red blood cell count could be important biological markers of metabolic syndrome and hyperuricemia in adolescents.

Funding: None; Competing interests: None stated.

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