Our objective was to assess whether serum angiotensin-converting enzyme (ACE) activity during routine clinical practice accurately reflects patient adherence to ACE inhibitor treatment in chronic heart failure (CHF). During 1994-95, ACE was measured in 73 CHF patients who were routinely attending the heart failure clinic at Ninewells Hospital. At the same time, the Medicines Monitoring Unit collected data on whether and when prescriptions for ACE inhibitors were redeemed at community pharmacies, which enabled each patient's adherence over a prolonged period to be assessed. We then correlated whether an elevation in serum ACE was associated with poor adherence with ACE inhibitor treatment. In total, 18% of CHF patients appeared to exhibit less than 70% adherence with their ACE inhibitor treatment, with 34% exhibiting less than 85%adherence and 58% exhibiting less than 100% adherence. A serum ACE activity of more than 12 U/L gave 91% positive predictive accuracy that the patient was less than 100% adherent with his or her ACE inhibitor treatment. At the other extreme, a serum ACE less than 6.5 U/L gave 81% positive predictive accuracy that the patient was more than 85% adherent with ACE inhibitor treatment. Nonadherence with ACE inhibitor treatment, therefore, was found to be common in patients with CHF. The simple, inexpensive test of serum ACE activity can be used in CHF patients to identify many, although not all, nonadherent patients so that adherence-enhancing strategies can be targeted toward them. Further work is clearly required to explore the precise clinical utility of this promising test. (c)2001 by CHF, Inc.