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Results from nearly 70% of routine lab tests in children varied with body weight, according to a new study.

In an analysis of 35 routinely assessed biochemical markers in blood tests, averages for about one-third differed significantly by the presence of overweight or obesity in the child, reported Victoria Higgins, PhD, of the Hospital for Sick Children and the University of Toronto, and colleagues in the .

Most affected were liver enzymes, lipids, and inflammatory markers, among them alanine aminotransferase, apolipoprotein B, complement factors C3 and C4, cholinesterase, high-sensitivity C-reactive protein, gamma-glutamyl transferase, haptoglobin, HDL cholesterol, iron, transferrin, triglycerides, and uric acid.

After adjusting for age and sex, these markers were significantly elevated according to BMI -- highest among children with obesity. One exception was iron, which was decreased with overweight or obesity.

Uric acid showed especially pronounced associations with increasing BMI. In both male and female participants, levels averaged 237 μmol/L, 261 μmol/L, and 270 μmol/L in normal weight, overweight, and obese children, respectively.

"Our study suggests that excess adiposity is positively associated with uric acid levels in a healthy population of children and adolescents, and thus a patient's weight status should be considered when interpreting uric acid results," the researchers recommended.

Several additional biomarkers were also significantly mediated by at least one measure of adiposity -- waist circumference or waist-to-height ratio -- but not necessarily affected by BMI category. These included albumin measured via the bromocresol purple method, alkaline phosphatase, apolipoprotein A1, amylase, aspartate aminotransferase, bilirubin direct, bilirubin total, lactate dehydrogenase, phosphorus, prealbumin, and total protein.

"We performed the first comprehensive analysis of the effect of obesity on routine blood tests in a large community population of children and found that almost 70 percent of the blood tests studied were affected," Higgins explained in a statement. "As clinical decisions are often guided by normative ranges based on a large healthy population, understanding how and which routine blood tests are affected by obesity is important to correctly interpret blood test results."

"We hope our study results will assist pediatricians and family physicians to better assess children and adolescents with different degrees of overweight or obesity," she added.

Data on more than 1,300 Canadian children and adolescents were included, ranging from 5 to 19 years of age. BMI values ranged from 13.4 to 65, although the majority of the participants were considered of normal weight, defined as from the 3rd to 85th percentile; 286 were overweight, and 136 were obese. The 97th percentile was the threshold for obesity.

"It is important to note that 31% of biochemical markers examined were not significantly different between BMI groups and were not significantly associated with any measures of adiposity," Higgins's group highlighted. These included anti-streptolysin O, calcium, cholesterol, blood CO2, creatinine, IgA, IgG, IgM, lipase, magnesium, and urea.

The findings point up to the wide-ranging metabolic impact from overweight and obesity even in childhood.

"[T]he majority of biomarkers exhibited differences by weight category, indicating that subclinical disease and metabolic abnormalities occur at an early age," they explained. "While it is unknown whether altered levels of biochemical markers in subjects with excess adiposity reflect health or indolent disease, it is important that altered analyte concentrations in subjects with overweight and/or obese are not normalized and that clinicians are aware of the effect of weight status on several laboratory tests."

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