Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Our findings indicate that a BNP level of 24.4 to 31.6 pg/ml is useful as an indication for surgery in ASD, VSD, and PDA patients; these results, in line with previous studies, make BNP a strong candidate as a predictive tool for early intervention.16,17 It is worth noting that a very recent study revealed that plasma BNP levels are elevated in children with heart disease before surgical treatment.32
 BNP concentration has been shown to be associated with the presence of a haemodynamically significant PDA in premature infants and to decrease to normal levels with ductal closure.33-36 In our study, which included older children with PDA, we found that patients with PDA and either a significant or a non-significant shunt always had significantly higher plasma BNP levels than all other groups (not only controls, but also patients with ASD and VSD).
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