I agreed with Betadine's Opinion - BNP is not an important param

 BNP is not an important parameter to diagnose heart failure.

Although Brain Natriuretic Peptide can be one of the indicators for heart failure, while here it was more likely for the doctor to know whether the perfusion volume was proper or just good enough for this patient since the dialysis machine often requires a large blood volume to go though.

When atrium of a heart got overloaded, the atrium would release Atrial Natriuretic Peptide (ANP) to help diuretic. When ventricle of a heart got overloaded, then ventricle would release Brain Natriuretic Peptide (BNP). The doctor wanted to know weather the perfusion volume was O.K. since the patient had edema at lower extremeries. If the doctor really wanted to find out heart failure for this patient, he should have had got an ultrasonic technician to measure the the remaining volume of the left ventricle and the ejection percentage.

For dialysis patient, BNP is not a good parameter to diagnose heart failure, rather, a good parameter for the perfusion load. The doctor measured BNP for his technique need.