(2000) 12, 197±204 ß 2000 Macmillan Publishers Ltd All rights reserved 0955-9930/00 $15.00 www.nature.com/ijir Measurement of erectile dysfunction in population-based studies: the use of a single question self-assessment in the Massachusetts Male Aging Study CA Derby 1 *, AB Araujo 1, CB Johannes 1, HA Feldman 1 and JB McKinlay 1 1 New England Research Institutes, Watertown, MA 02172, USA A concise, reliable means of assessing erectile dysfunction (ED) in large, multidisciplinary population-based studies is needed. A single, direct question for self-assessed ED was assessed in the population-based sample of the Massachusetts Male Aging Study (MMAS). Of the 1156 respondents to the 1995 ± 97 MMAS follow-up evaluation, 505 were randomly selected to complete either the International Index of Erectile Function (IIEF) (n ˆ 254), or the Brief Male Sexual Function Inventory (BMSFI) (n ˆ 251), in addition to the single question self-assessment. The proportion not classi ed due to missing data was MMAS ± 9%, BMSFI ± 8%, and IIEF ± 18%. The single question correlated well with these other measures (r ˆ 0.71 ± 0.78, P < 0.001). Prevalence was similar to that based on the IIEF, agreement was moderate (kappa ˆ 0.56 ± 0.58), and associations with previously identi ed risk factors were similar for each classi cation. Thus, the MMAS single question may be a practical tool for population-based studies where detailed clinical measures of ED are impractical. (2000) 12, 197±204. Keywords: erectile dysfunction; impotence; population studies; sexual function; epidemiologic methods Introduction Although erectile dysfunction (ED) is estimated to affect millions of men in the USA, little is known regarding the epidemiology of this physiologically important condition in a general population. 1 Epidemiological studies of ED are limited by the lack of consistent de nitions and assessment tools suitable for use in population-based samples. While the 1993 NIH consensus conference de nition highlights the subjective nature of ED, there remains no gold standard subjective measure. 1 The available questionnaires were developed in clinical settings for self-selected populations seeking treatment, or in clinical trials of therapies, and their applicability to epidemiologic surveys in population-based samples is limited. These instruments have not been standardized in random sample population-based studies. In addition, they increase respondent burden *Correspondence: CA Derby, New England Research Institutes, 9 Galen Street, Watertown, MA 02172, USA. E-mail: cderby@neri.org Received 19 July 1999; accepted 19 April 2000 with multiple detailed queries on a sensitive topic, and often cannot be scored due to incomplete responses (which is common for questionnaires regarding sensitive topics). 2±7 Thus, there is need for an appropriate method for assessing ED in population-based studies. Clinical studies require instruments that are highly sensitive to detect changes with treatment, or to optimize case nding where screening is followed by clinical evaluations. In contrast, large multidisciplinary surveys require instruments that accurately estimate the presence or absence of ED while minimizing logistical complexity and respondent burden. The Massachusetts Male Aging Study (MMAS) is a multidisciplinary prospective cohort study of health and aging in a random sample of men residing in the Boston area. 8±10 The follow-up evaluation included a con dential self-administered sexual activity questionnaire that contained a single question method for determining ED status. Respondents were rst given a concise de nition of impotence, and then asked to specify which of four clearly de ned categories best described their status. The purpose of this analysis is to compare the use of this single, direct, subjective question in a large population-based epidemiological study with two indices previously developed and validated in clinical settings.
198 Materials and methods Study population The MMAS baseline surveys were conducted from 1987 to 1989 in a random sample of men residing in 11 cities and towns in the vicinity of Boston, MA. Of the 1709 respondents at baseline, 180 were con rmed as deceased, 28 were ineligible due to serious illness, and ve had moved from the country. The remaining 1496 men were alive and eligible for follow-up, and 1156 (77%) completed a follow-up evaluation between 1995 and 1997. As described below, the present study is based on data from the 505 follow-up interviews conducted between 1 January and 30 September, 1997. Details of the MMAS study design have been described previously. 8±10 In brief, trained interviewer=phlebotomists visited subjects in their homes and completed a health questionnaire, psychological instruments, physiological measurements and blood sampling. A self-administered sexual activity questionnaire was completed in private and returned in a sealed envelope. At follow-up, the overall response rate for the sexual activity questionnaire was 96% (1104=1156). In order to validate this questionnaire and to evaluate comparability with other more widely used instruments, respondents interviewed after 1 January, 1997 were randomly assigned to receive a sexual activity questionnaire which included either the Brief Male Sexual Function Inventory (BMSFI), 3 (n ˆ 251), or the International Index of Erectile Function (IIEF) 2 (n ˆ 254). According to the MMAS protocol for the sexual activity questionnaire, these instruments were con- dential and self-administered. A total of 505 respondents were interviewed during this period, and constitute the analysis population. All analyses are based on data from these follow-up questionnaires, in which ED status was assessed in two ways, according to the MMAS single question, and as de ned by either the IIEF or the BMSFI (Figure 1). Classi cation of ED status The MMAS single question for de ning ED status includes a short de nition of erectile dysfunction (impotence) followed by four clearly de ned categories of ED ranging from no dysfunction to complete dysfunction. Men were asked to indicate which of these categories best described their current condition (Table 1). For this analysis two dichotomous de nitions of ED were constructed. Men were de ned as having moderate to complete ED if they classi ed themselves as either moderately or completely impotent. Men were classi ed as having any ED if they classi ed themselves as having minimal, moderate or complete impotence. The BMSFI and IIEF instruments were scored according to published protocols. 2,3 The IIEF instrument consists of 15 items, six of which are included in the erectile function domain (Table 1). The sum of these items results in a quantitative score ranging from 1 to 30. This scale is then divided into ve categories, representing no dysfunction (25 ± 30), mild dysfunction (19 ± 24), mild-to-moderate dysfunction (13 ± 18), moderate dysfunction (7 ± 12) and severe dysfunction (1 ± 6). 2 Two dichotomous classi cations of ED analogous to those described above were then created for purposes of this analysis. Moderate to severe ED was de ned as a score of 12, while any ED was de ned as a score of 24. The BMSFI consists of 11 items of which three make up a domain for erectile function (Table 1). 3 This instrument does not specify categorical cutpoints, but is rather a quantitative measure ranging from 0 (complete lack of erectile ability) to 12 (normal function). Figure 1 Selection of analysis population from among respondents to the follow-up evaluation of the Massachusetts Male Aging Study, Boston, MA, 1995 ± 1997.
Table 1 Questions pertaining to erectile function in MMAS, IIEF, and BMSFI instruments, Massachusetts Male Aging Study, Boston, MA, 1995 ± 1997 199 I. Massachusetts Male Aging Study: Single Question Assessment of Erectile Dysfunction: IMPOTENCE means not being able to get and keep an erection that is rigid enough for satisfactory sexual activity. How would you describe yourself? Not impotent: always able to get and keep an erection good enough for sexual intercourse. Minimally impotent: usually able to get and keep an erection good enough for sexual intercourse. Moderately impotent: sometimes able to get and keep an erection good enough for sexual intercourse. Completely impotent: never able to get and keep an erection good enough for sexual intercourse. II. International Index of Erectile Function: Questions Included in Erectile Function Domain, Rosen et al. 2 Over the past 4 weeks: 1. How often were you able to get an erection during sexual activity? 2. When you had erections with sexual stimulation, how often were your erections hard enough for penetration? 3. When you attempted sexual intercourse, how often were you able to penetrate (enter) your partner? 4. During sexual intercourse, how often were you able to maintain your erection after you had penetrated (entered) your partner? 5. During sexual intercourse, how dif cult was it to maintain your erection to completion of intercourse? 6. How do you rate your con dence that you can get and keep your erection? Response Set: 6-item likert scale (0 ± 5). III. Brief Male Sexual Function Inventory: Questions Included In Erectile Function Domain, O'Leary et al. 5 In the past 4 weeks: 1. How often have you had partial or full sexual erections when you were sexually stimulated in any way? 2. When you had erections, how often were they rm enough to have sexual intercourse? 3. How much dif culty did you have getting an erection? Response set: 5-item likert scale (0 ± 4) Statistical methods Descriptive characteristics of respondents to each of the questionnaires were compared using chi-square statistics and analysis of variance. The ordinal responses to the MMAS single question and scores from the IIEF and BMSFI were compared using Spearman correlation coef cients. Analysis of variance was used to compare the scores for the IIEF and BMSFI across categories of the MMAS single question. Agreement between dichotomous ED classi cations based on the MMAS single question and the IIEF were assessed with kappa. Finally, logistic regression was used to evaluate whether ED as de ned by the single question and by the IIEF showed similar associations with previously reported risk factors for the condition. Results Characteristics of men who received the IIEF, BMSFI and those in the follow-up sample who were not included in the validation study are compared in Table 2. Respondents not included in the validation study were more likely to have at least one sexual partner (w 2 2 ˆ 7.3, P < 0.05). Men in the IIEF sample had a slightly lower level of education. Otherwise, the validation samples and the remainder of the MMAS follow-up sample were comparable with respect to variables potentially associated with ED status. The response rates were similar for each instrument, ranging from 96% for the MMAS single question to 94% and 93% for the BMSFI and IIEF, respectively. In contrast, the proportion of respondents for whom ED status could not be classi ed due to missing data varied substantially by questionnaire (Table 3). The proportion of respondents unable to be classi ed with the MMAS single question (9%) was similar to the proportion with incomplete data for the BMSFI (8%), and half the proportion unable to be classi ed with the IIEF (18%). For each questionnaire, men with missing ED status tended to be older, and to have a lower level of education and income although differences were not statistically signi cant. The prevalence of moderate to severe ED as determined by the MMAS single question was 25% (95% con dence interval (95% CI) 20 ± 29%). This was the same as the 25% prevalence (95% CI 19 ± 32%) of moderate or severe dysfunction derived using the IIEF (Figure 2). Prevalence estimates for any ED were also the same, 50% (95% CI 45 ± 54%) for the MMAS single question and 50% (95% CI 42 ± 57%) for the IIEF. Prevalence estimates based on each de nition also showed similar sequential increases when analysed by decade of age. Because the BMSFI results in a continuous score, prevalence estimates were not calculated for this instrument. The MMAS single question showed moderately high correlations with both the BMSFI and the IIEF. The Spearman correlation coef cients for the four ordinal responses to the MMAS with the summary scores from the BMSFI and IIEF were ± 0.78 and ± 0.71, respectively, (P < 0.001). Mean scores for the
200 Table 2 Selected characteristics of follow-up respondents by validation sample group, Massachusetts Male Aging Study, Boston, MA, 1995 ± 1997 Validation sample group Attribute IIEF BMSFI Not in validation sample n 254. 251. 651. Mean Age, years (s.d.) 62.6 (8.1) 62.5 (8.6) 62.4 (8.5) Education, (n)% < High school (34) 13.5 (18) 7.2 (48) 7.4 High school (38) 15.1 (35) 13.9 (100) 15.5 > High school (180) 71.4 (198) 78.9 (498) 77.1 Self-reported history of Heart disease, (n)% (43) 16.9 (34) 13.6 (108) 16.6 Hypertension, (n)% (75) 29.5 (81) 32.0 (233) 36.0 Diabetes, (n)% (24) 9.5 (26) 10.4 (52) 8.0 Mean BMI, kg=m 2 (s.d.) a 27.5 (4.7) 27.1 (4.2) 28.0 (4.5) Smoking status (n)% Never (62) 24.5 (57) 22.8 (140) 21.6 Prior (153) 60.5 (163) 65.2 (433) 66.7 Current (38) 15.0 (30) 12.0 (76) 11.7 Alcohol use > 3 drinks=day (n)% (35) 13.8 (36) 14.3 (75) 11.5 At least one sexual partner (n)% b (167) 77.7 (162) 72.7 (458) 81.4 Intercourse in past 6 months (n)% c (146) 68.9 (143) 67.5 (383) 67.2 BMIˆ body mass index; BMSFI ˆ Brief Male Sexual Function Inventory; IIEF ˆ International Index of Erectile Function; s.d. ˆ standard deviation. a Due to missing data, n for this variable are: IIEF n ˆ 233, BMSFI n ˆ 228, not in validation sample n ˆ 607. b Due to missing data, n for this variable are: IIEF n ˆ 215, BMSFI n ˆ 223, not in validation sample n ˆ 563. c Due to missing data, n for this variable are: IIEF n ˆ 212, BMSFI n ˆ 212, not in validation sample n ˆ 570. Table 3 Proportion of sample with missing responses, by instrument, Massachusetts Male Aging Study, Boston, MA, 1995 ± 1997 Instrument Number of items nin sample n (%) with all items unanswered n (%) some items answered, but incomplete items for ED classi cation Total n (%) with ED status not classi ed due to missing responses MMAS sexual activity questionnaire 1 505 19 (4) 28 (6) 47 (9) BMSFI 3 251 15 (6) 4 (2) 19 (8) IIEF 6 254 17 (7) 29 (12) 46 (18) BMSFI ˆ Brief Male Sexual Function Inventory; ED ˆ erectile dysfunction; IIEF ˆ International Index of Erectile Dysfunction; MMAS ˆ Massachusetts Male Aging Study. Figure 2 Prevalence of moderate to severe erectile dysfunction (ED) and of any ED as de ned by the Massachusetts Male Aging Study (MMAS) single question and the International Index of Erectile Dysfunction (IIEF), Massachusetts Male Aging Study, Boston, MA, 1995 ± 1997. IIEF and BMSFI within categories of the MMAS single question are shown in Figure 3. The single question explained 61% of the variance in BMSFI scores (ANOVA P < 0.001), and 50% of the variance in IIEF scores (ANOVA P < 0.001). Figure 4 displays comparisons of dichotomous ED classi cations according to the MMAS single question with those based on the IIEF. Kappa for these two measures was 0.58 (95% CI 0.44 ± 0.71) for moderate to severe ED, and was 0.56 (95% CI 0.45 ± 0.68) for any ED. In general, the disagreement between the single question and the IIEF was due to men being classi ed as having ED by the IIEF and as not having ED according to the single question. Disagreement in the opposite direction was minimal (Figure 4). Table 4 shows the cross-sectional relationship of ED to selected factors that have been associated with ED in previous studies. In general, although the
201 Figure 3 Mean scores for the International Index of Erectile Function (IIEF) and Brief Male Sexual Function Inventory (BMSFI) by erectile dysfunction (ED) category for the Massachusetts Male Aging Study (MMAS) single question, Massachusetts Male Aging Study, Boston, MA, 1995 ± 1997. point estimates varied somewhat, the overlapping con dence intervals suggest that associations were similar for ED de ned by the MMAS single question and the IIEF. The odds ratio for ED increased with age, with the relative odds for men in their 60s 2 ± 3 times greater than that for men in their 50s, and the relative odds for men aged 70 ± 79 y 6 ± 10 times that of men in their 50s. The association between ED as de ned by the MMAS single question and history of hypertension or diabetes was similar to the association estimated for ED de ned using either of the other assessments. The estimated associations with smoking and BMI were also similar regardless of the questionnaire used to de ne ED. Discussion Currently available ED questionnaires were originally developed and evaluated in clinic-based samples. While these instruments have been used in large numbers of men screened for participation in clinical trials, 25 to our knowledge, their appropriateness for use in large population-based epidemiologic studies of non-patients has not been tested. In a clinical setting, detailed information regarding erectile function is required to de ne degree of dysfunction or to detect changes with treatment. In contrast, in population-based studies where ED is Figure 4 Agreement between classi cations of erectile dysfunction (ED) based on the Massachusetts Male Aging Study (MMAS) single question and the International Index of Erectile Function (IIEF), Massachusetts Male Aging Study, Boston, MA, 1995 ± 1997. CI ˆ con dence interval.
202 Table 4 Age-adjusted odds ratios for erectile dysfunction (95% con dence intervals) for selected characteristics by instrument, Massachusetts Male Aging Study, Boston, MA, 1995 ± 1997 Moderate=severe ED Any ED Characteristic MMAS IIEF MMAS IIEF Age (y) 50 ± 59 1.0 1.0 1.0 1.0 60 ± 69 3.8 (2.1 ± 7.1) 1.9 (0.9 ± 4.3) 1.9 (1.2 ± 2.9) 2.4 (1.3 ± 4.6) 70 ± 79 11.2 (5.9 ± 21.0) 6.0 (2.5 ± 14.3) 4.9 (2.9 ± 8.3) 6.8 (2.9 ± 15.9) History of a Heart Disease 2.5 (1.4 ± 4.5) 1.6 (0.7 ± 3.7) 1.3 (0.7 ± 2.3) 0.9 (0.4 ± 2.1) Hypertension 1.8 (1.1 ± 3.0) 2.4 (1.2 ± 4.9) 1.5 (1.0 ± 2.3) 3.3 (1.7 ± 6.6) Diabetes 4.9 (2.4 ± 10.0) 3.4 (1.2 ± 9.9) 4.2 (1.9 ± 9.3) 2.9 (0.9 ± 9.0) Smoking Never 1.0 1.0 1.0 1.0 Prior 0.9 (0.5 ± 1.6) 0.6 (0.3 ± 1.4) 0.9 (0.5 ± 1.4) 1.4 (0.7 ± 2.8) Current 1.4 (0.7 ± 3.1) 1.9 (0.7 ± 5.4) 0.9 (0.4 ± 1.7) 4.1 (1.4 ± 11.9) Overweight a (BMI 27.8 kg=m 2 ) 1.7 (1.0 ± 2.7) 1.2 (0.6 ± 2.4) 1.6 (1.0 ± 2.5) 1.7 (0.9 ± 3.2) BMIˆ body mass index; ED ˆ erectile dysfunction; IIEF ˆ International Index of Erectile Dysfunction; MMAS ˆ Massachusetts Male Aging Study single question. a Referent category is men without the condition or characteristic. not the only outcome of interest and the objective is a dichotomous classi cation of ED status (ie, present or absent), these clinical assessment tools may be inappropriately detailed and lengthy. This is particularly true in lengthy, multifaceted comprehensive health surveys where respondent burden can be quite high. The screening characteristics required of a questionnaire depend upon the population to which it will be applied. 11 In clinical settings, an ED screening questionnaire that is highly sensitive may be desirable given that men are reevaluated using more speci c clinical criteria. In contrast, in a populationbased study that does not include follow-up clinical evaluations, sensitivity must be balanced with speci city in order to minimize misclassi cation. In the absence of a gold standard, the sensitivity and speci city of the MMAS question, IIEF and BMSFI cannot be directly compared. This analysis showed that relative to these other measures, the single question is less likely to classify men as having moderate to complete ED (percent positive agreement (63%)). Conversely, there was high agreement for classifying men as not having ED (percent negative agreement (93%)). This difference could be due to under-reporting bias with the MMAS single question. However, Conte has reviewed self-assessments for sexual functioning and concluded that despite the in uence of response biases, self-report is meaningful when questions are asked directly with clear response choices. 5 Because they represent a constellation of perceptions weighted according to the values and preferences of the individual, self-ratings provide unique information that cannot otherwise be obtained. 12 The utility of a single, clearly stated question for assessing aspects of health status has been previously reported. Longitudinal studies have consistently demonstrated that self-rating of global health status predicts mortality 12 and single questions have been shown to be useful for classifying physical activity status 13 and for identifying depression. 14 Wide variation has been demonstrated in the extent to which men perceive various health states as problematic. 15 Thus, a potential disadvantage of the single question approach is that compared with more detailed assessments, a single subjective question may be more heavily in uenced by the respondent's knowledge, perceptions and expectations. For example, ED is often accepted as an inevitable consequence of aging, although men are increasingly seeking treatment with the expectation of returning to previous levels of function. 1,16 ± 19 An advantage of the MMAS single question is that a global question regarding self-assessment of ED status is consistent with the view among urologists that the patients' subjective assessment is critical to de ning ED. 1 Furthermore, logistic regression analyses showed that associations between ED and potential risk factors were qualitatively similar for each of the ED measures studied, and were consistent with previously reported associations. 1,18,20 ± 24 This suggests that the MMAS single question de nition of ED is consistent with previously reported de nitions and may be useful in epidemiologic studies. Another advantage of the MMAS single question is that men can respond regardless of whether they have recently had sexual intercourse. Sexual function questionnaires often include items worded such that they apply only to men who have been sexually active in the recent past. 3 This limitation restricts the proportion of respondents who can complete the questionnaire, particularly as the age of the study population increases. Overall, one third of the 505 men in this analysis reported that they had had no intercourse during the past 6 months.
This proportion increased with decade of age, with no intercourse reported by 21%, 31% and 55% of men aged 50 ± 59, 60 ± 69 and 70 ± 79 y respectively. Another problem with previously described sexual activity questionnaires is that the scoring includes the implicit assumption that lack of sexual activity indicates the presence of ED. Although lack of activity may in fact be indicative of ED, this assumption may not be valid particularly in an aging population where co-morbidity, and the availability and health status of a partner rather than functional capabilities may also explain a lack of sexual activity. Relative to multi-item indices such as the IIEF and BMSFI, the MMAS single question may minimize the proportion of men for whom ED status cannot be determined due to missing data. Men seeking treatment for complaints of ED may be willing to answer detailed questions regarding their sexual function. However, given the sensitive nature of these questions, it is not surprising that nonresponse rates are high in a population-based sample. The proportion with missing data for the MMAS single question was nearly half that for the IIEF, and similar to that for the BMSFI. The present study was not designed to test whether the single question reduces missing information. In fact, the present study design may have overestimated the proportion of men who choose not to answer a single question regarding self-assessed ED status. The MMAS question appeared at the end of a 23- question sexual activity questionnaire. Men who were adversely in uenced by these questions may have stopped answering before getting to the single ED question. Non-response may have been further exacerbated given that men in this validation study were asked to complete a second sexual function inventory. Thus, the MMAS single question might prove even more effective if presented in the context of a general health and medical history inventory, where previous questions are less likely to in uence the probability that a respondent will answer. Finally, because it is straightforward and direct, the MMAS single question may be appropriate for use in cross-cultural studies. Little is known regarding the distribution of ED across geographic regions, cultures and ethnic groups. 1 Valid comparisons across populations require a simple consistent de nition of ED assessed with an instrument that is easily translated, administered and scored. Problems of missing data and cultural acceptability encountered with more detailed evaluation tools may be ampli ed when applied in different social contexts. Recently, the MMAS single question has been used successfully in a multinational population-based study conducted in Japan, Malaysia, Brazil and Italy. 26 There are several limitations to the current analysis. First, as mentioned above, because the MMAS single question was included in a multi-item sexual activity questionnaire, the proportion of missing responses may be overestimated. Because there is no gold standard for evaluating ED, the sensitivity and speci city of the different screening questionnaires cannot be evaluated. The current study is therefore limited to measures of agreement between the MMAS single question and the IIEF and BMSFI. The current results are based on a predominantly white sample of men aged 50 ± 79 y and may not be generalizable to populations with different demographic characteristics. The validity and cultural acceptability of the question needs to be evaluated in a multi-ethnic sample and across a broader age range. The single question has been used successfully in a recent study comparing population samples of men in four countries. 26 The MMAS single question is appropriate for use in population-based studies where a precise classi- cation of degree of dysfunction is not the primary focus, and inclusion of detailed, multi-item indices on a sensitive topic is not feasible. This measure would not be appropriate for clinical studies in which ED is the primary outcome of interest, or in which more detailed clinical assessments are required to detect changes with treatment. It is important to note that the original MMAS single question included the term `impotence', rather than `erectile dysfunction'. At the time that the MMAS follow-up questionnaire was developed, the 1993 NIH consensus conference recommendations 1 were relatively new, and the term erectile dysfunction was not widely used, particularly among persons in the general population. For this reason, the original questionnaire used the term impotence. We acknowledge that the term erectile dysfunction is preferable, and is now more widely used. Therefore, for future studies, we would advocate rephrasing the single question by replacing the term `impotence' with `erectile dysfunction, (sometimes called impotence)'. The resulting question would be entirely consistent with the NIH consensus conference de nition. In summary, the MMAS single direct question for self-assessment of ED status provides a practical and valid means for assessing ED status in large population-based studies where multiple health outcomes are measured. Compared with multi-item scales for determining ED status, the MMAS single question may reduce the proportion of men unable to be classi ed due to item non-response. The MMAS single question is well correlated with both the IIEF and BMSFI indices. The applicability of the single question is not dependent on respondents having had sexual intercourse in the recent past, and does not include the implicit assumption that lack of intercourse is indicative of ED. Finally, associations between the MMAS single question de nition of ED and age, history of chronic diseases, smoking, and weight are consistent with previously reported associations. 203
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