Background: Brain natriuretic Peptide (BNP) is been used as a standard diagnostic tool to define heart failure. However, the significance of inappropriate levels of plasma BNP in the presence or absence of heart failure remains unclear. We evaluated the prevalence and clinical characteristics of patients with inappropriate BNP level. Methods: Of 3830 patients who measured BNP levels from Aug 2002 to Jun 2003 (mean age: 62.8짹3.8, male 49%), 1048 patients were analyzed (control 661, systolic heart failure 387 patients). We defined systolic heart failure was EF <50% and LVEDD >5.5cm with typical symptoms. BNP (Triage짰, Biosite) and routine chemicals were analyzed in blood samples obtained on admission or at first visit. Assessment was by clinical examination, resting 12 lead ECG, chest x-ray, blood and urine analysis and echocardiography. We defined that false positive test was elevated BNP (>100 pg/ml) with no other non-cardiac causes of elevated BNP and false negative test was not elevated BNP (<100 pg/ml) with definite LV systolic dysfunction. Patients with isolated left ventricular diastolic dysfunction were excluded. Results: At 100 pg/ml, BNP had a sensitivity of 92.5% and specificity of 86.1%. 166 patients had inappropriate levels of BNP in the presence or absence of systolic heart failure. Of 166 patients (average age 73.8짹9.3 years, 57% female), 72% had false positive test and 28% had false negative test. Patients with false negative test had a dilated cardiomyopathy (52.1%), stable CHF status (NYHA I, 26.2%) and previous MI history (12%). Patients with false positive test had a history of stable CHF (27.9%), hypertension (21.0%), diabetes mellitus (19.7%), valvular heart disease (15.3%), atrial fibrillation (11.23%). These patients were significantly older (average age 73.8짹8.7 vs. 65.8짹8.8 years, p=0.001) than control group (BNP> 100pg/ml with decreased EF), with all other demographic and clinical criteria being similar. Conclusions: In BNP test for congestive heart failure, there was a gray zone as diagnostic tools in congestive heart failure. But. These patients will need a follow up for the relation to the clinical outcomes.