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Background: Recent UNAIDS guidelines recommend measuring concurrency 6 months before the interview date, based on overlapping partnership dates. This has theoretical advantages, but little is known about how well it can be measured in practice. Methods: The assumptions underlying the UNAIDS measure were tested using data from a sexual behaviour survey conducted in rural northern Malawi. All resident adults aged 15-59 were eligible. Questions included self-reported concurrency and dates for all marital and nonmarital partnerships in the past 12 months. Results: A total of 6796 women and 5253 men were interviewed, 83 and 72% of those eligible, respectively. Since few women reported multiple partners, detailed analysis was restricted to men. Overall 19.2% [95% confidence interval (CI) 18.1-20.2] of men self-reported concurrent relationships in the past year (almost all of those with more than one partner). Using overlapping dates the estimate was 16.7% (15.7-17.7). Excluding partnerships which tied on dates (making overlap uncertain) or restricting the analysis to the three most recent partners gave similar results. The UNAIDS 6-month measure was 12.0% (11.1-12.9), and current concurrency was 11.5% (10.6-12.4). The difference between dates-based and self-reported 12-month measures was much larger for unmarried men: 11.1% (9.7-12.4) self-reported; 7.1% (6.9-8.2) on dates. Polygyny (15% of married men) and the longer duration of relationships stabilized the estimates for married men. Nonmarital partnerships were under-reported, particularly those starting longer ago. Conclusions: The difficulties of recall of dates for relationships, and under-reporting of partners lead to underestimation of concurrency using date-based measures. Self-reported concurrency is much easier to measure and appears more complete.

Objective: To investigate the relationship between changing morbidity patterns, the use of hospital services by HlV-infected patients and the uptake of antiretroviral therapy (ART) in England. Design: Prospective serial cross-sectional analyses based on data collected through the National Prospective Monitoring System (NPMS), a multi-centre prospective monitoring system. Setting: HlV-infected patients seen in 10 clinics, five London and five non-London, during the three semesters, 1 January 1996 to 30 June 1997. Main outcome measures: The mean use of hospital services per patient-year, mean new HlV-related opportunistic illnesses per 1000 patient-years and percentage uptake of ART. Results: The use of inpatient services changed particularly among AIDS patients. The mean number of inpatient days for AIDS patients decreased from 19.7 [95% confidence interval (Cl) 13.7-25.7] in 1996 to 11.2 (95% Cl 6.1-15.6) per patient-year in 1997. Concurrently the number of new AIDS-defining events decreased significantly from 567 (95% Cl 529-607) to 203 (95% Cl 183-225) per 1000 patient-years. The overall uptake of ART increased significantly from 33% (95% Cl 31-35%) to 50% (95% CI 48-52%), and a switch from mono or dual to triple therapy or quadruple or more therapy was observed. However, by mid-1997 only 29% (95% Cl 26-32%) of asymptomatic patients and 51% (95% Cl 49-54%) of patients with symptomatic non-AIDS were on ART, compared with 69% (95% Cl 66-71%) of AIDS patients. Conclusion: The observed reduction in new AIDS-defining events has led to a reduction in the need for inpatient hospital care and has been associated with an increased uptake of ART, including a switch to triple therapy. All of these factors are likely to have contributed to the observed reduction in mortality among English AIDS patients. As the overall uptake of ART remained relatively low in English centres further improvements can be anticipated. However, the medium to long-term effects of these treatment regimens will need to be closely monitored.