Responsiveness and Clinically Important Differences for the VF-14 Index, SF-36, and Visual Acuity in Patients Undergoing Cataract Surgery Amaia Bilbao, MSc,1 José M. Quintana, MD, PhD,2 Antonio Escobar, MD, PhD,3 Susana García, MD, MSc,2 Elena Andradas, MD, PhD,4 Marisa Baré, MD, PhD,5 Belén Elizalde, MD, MPH,6 for the IRYSS-Cataract Group Objective: To assess visual acuity (VA) and 2 questionnaires of health-related quality of life—the Visual Function 14 (VF-14) index and the Medical Outcomes Study Short Form 36 Health Survey (SF-36)—as instruments for capturing clinically important changes after cataract surgery. Design: Prospective, observational study. Participants: Four thousand three hundred fifty-six consecutive patients attending ophthalmologic clinics in 17 hospitals in preparation for cataract surgery were recruited. Methods: Clinical data were collected in the visit before the intervention and 6 weeks after surgery by ophthalmologists. Patients completed the questionnaires before surgery and 3 months after surgery. Main Outcome Measures: The VF-14 and SF-36 questionnaire results obtained before surgery and 3 months after the procedure and VA before the procedure and 6 weeks afterward. Results: Positive mean changes in VA (⫹0.47) and VF-14 results (⫹24.03) indicated significant improvements after cataract surgery that were not reflected in changes in SF-36 domains (from 1.86 to 5.62). Responsiveness parameters demonstrated large changes in VA and VF-14 scores but not in SF-36 domains. The minimal clinically important differences (MCID) after surgery were 0.41 for VA and 15.57 for VF-14 results; the minimal detectable change (MDC) for VF-14 was 10.81. Conclusions: Visual acuity and VF-14 scores, but not SF-36 scores, are appropriate instruments for capturing clinically important changes after cataract surgery. The MCID and MDC values obtained herein, although not absolute thresholds, may aid in the interpretation of changes in VA and VF-14 scores. Financial Disclosure(s): The author(s) have no proprietary or commercial interest in any materials discussed in this article. Ophthalmology 2009;116:418 – 424 © 2009 by the American Academy of Ophthalmology. Ophthalmologists long have recognized that visual acuity (VA) does not sufficiently measure the visual impairment caused by cataract. More rigorous and effective instruments for functional assessment are needed to complement the assessment of clinical status measured by VA.1 Several vision-specific health-related quality of life (HRQoL) instruments have been used to evaluate functional impairment related to vision.2– 6 One of these, the Visual Function 14 (VF-14) index, developed by Steinberg et al,3 has gained increasing acceptance in the recent literature.7–18 To assess the potential usefulness of HRQoL tools in longitudinal evaluations, they should be evaluated for responsiveness to clinically significant changes. Responsiveness is the ability of a measure to detect small but clinically important changes in health status over time.19,20 The minimal clinically important difference (MCID) and the minimal detectable change (MDC) are related to responsiveness, but they are more clinically oriented and focused at the individual level. These 2 measures are important given that average effects across a group may not be meaningful to the individual patient.21 To the 418 © 2009 by the American Academy of Ophthalmology Published by Elsevier Inc. best of the authors’ knowledge, the MCID and MDC have not been evaluated with the VF-14 after cataract surgery. The literature supports the fact that vision-specific instruments should be complemented with generic HRQoL instruments.22 One of the most commonly used generic HRQoL instruments, the Medical Outcomes Study Short Form 36 Health Survey (SF-36),23 has been used by some authors in vision-related studies.11,18 To provide more information on changes in quality-oflife measures after cataract surgery, the authors measured VA along with VF-14 and SF-36 and determined the responsiveness, MCID, and MDC parameters in a large, prospective observational study of patients undergoing cataract surgery. Patients and Methods Data Collection Patients were recruited from 17 participating hospitals belonging to the Spanish National Health Service. In each hospital, conISSN 0161-6420/09/$–see front matter doi:10.1016/j.ophtha.2008.11.020 Bilbao et al 䡠 Responsiveness, MCID, and MDC after Cataract Surgery secutive patients attending ophthalmologic clinics for the evaluation of cataract removal surgery by phacoemulsification were invited to participate in the study. Patients with severe comorbidities, such as cancer or a terminal disease, or psychiatric conditions that precluded them from completing the questionnaire were excluded, as were those with corneal dystrophy. Physicians in each hospital were masked to the study goals. Recruitment of patients took place between October 2004 and July 2005. Clinical data were collected in the visit before the intervention and approximately 6 weeks after surgery by ophthalmologists and nurses collaborating in the study. These data were sociodemographic, data related to surgical technique and complications, and best-corrected visual acuity when appropriate, or uncorrected if not refractive ocular pathology, measured by Snellen optotypes. Three categories for VA were considered—ⱕ0.1, 0.2 to 0.4, and ⱖ0.5—as other authors have defined.24,25 The physicians and nurses were trained to retrieve the medical record data in similar ways and were provided with a manual that included the definitions of all variables and categories. At the time of the preintervention visit, all patients were mailed 2 HRQoL questionnaires, the SF-36 and VF-14, as well as additional questions about their ocular disease. To increase the response rate to the mailed questionnaires, up to 2 reminder letters were mailed at predetermined times to those patients who had not yet returned the questionnaires. Telephone calls were made when necessary to collect this information. The VF-14 is a measure of functional capacity related to vision based on 14 vision-dependent activities performed in everyday life that can be affected by cataract.3 The score on the VF-14 ranges from 0 (worst level of function) to 100 (best level of function). The validity and internal consistency of the VF-14 have been documented,3 and the instrument has been translated and validated in Spanish populations.26,27 The SF-36 covers 8 domains,23 with scores ranging from 0 to 100. Higher scores indicate better health status. The SF-36 has been translated into Spanish and validated in Spanish populations.28 Approximately 3 months after cataract removal, patients were sent another letter, copies of the VF-14 and SF-36, and additional questions regarding the clinical aspects of their disease. At this time, patients also were asked a transitional question about their improvement with regard to their vision after cataract removal, “How is your vision now compared with how it was before your cataract surgery?” with 7 answer options ranging from “a great deal better” to “a great deal worse.” Participants who did not return this information promptly were followed up as described previously. All study procedures were approved by the institutional review boards of the participating hospitals. Personal data were collected only for tracking patients during the period before and after surgery, and all data were kept confidential. Statistical Analysis The unit of study was the patient. When a study participant underwent 2 interventions during the study period, only the first 1 performed was included. Descriptive statistics include frequency tables, means, and standard deviations (SDs). The sociodemographic and clinical characteristics at baseline were compared between the responders and the nonresponders 3 months after surgery using the chi-square and Fisher exact tests for categorical variables and the Student’s t-test or the nonparametric Wilcoxon test for continuous variables. Means and SDs were calculated for VA as well as for the VF-14 and SF-36 domains before and after surgery. A paired t test was used for the comparison between before and after surgery. The analyses for the VA and VF-14 were performed according to VA categories at baseline (ⱕ0.1, 0.2– 0.4, ⱖ0.5) by means of the analysis of variance with Scheffé’s test for multiple comparisons. The comparison of VA and VF-14 according to ocular pathology (simple cataract vs. cataract with retinopathy or other pathology) was performed by means of the Student’s t-test. Ceiling and floor effects (a high proportion of patients scoring in the highest or the lowest possible scale score, respectively) at baseline and 3 months after surgery were examined for each domain to evaluate discrimination ability of the scales. Mean changes in VA as well as in VF-14 and SF-36 domains were compared across categories according to the transitional question about perceived improvement with regard to their vision after cataract removal surgery. For the comparison, the analysis of variance with Scheffé’s test for multiple comparisons or the nonparametric Kruskal-Wallis test was performed. To measure the responsiveness of VA and the 2 HRQoL instruments after surgery, the following indexes29 were used: standardized effect size, defined as the mean change score divided by the SD of baseline scores; standardized response mean, defined as the mean change score divided by the SD of changed scores; and Guyatt responsiveness index, defined as the mean change score of patients reporting “somewhat better” vision after surgery divided by the SD of change scores in stable patients. Stable patients were defined as those reporting no change on the transitional question. Cohen’s benchmarks were used to classify the magnitude of effect sizes: ⬍0.20, not significant; 0.20 to 0.49, small; 0.50 to 0.79, moderate; and ⱖ0.80, large.30 In the case of the VF-14, with the aim of assessing the effect of the better-seeing eye, responsiveness parameters also were estimated depending on if the better-seeing eye was the operated eye or the fellow one. The MCID was estimated for VA, as well as for VF-14 and SF-36 domains, by the mean change score for patients whose response to the transitional question was “somewhat better”.31 The MDC expresses the minimal magnitude of change above which the observed change is likely to be real and not just measurement error. For the estimation of MDC, the standard error of measurement (SEM), which represents the amount of error associated with an individual subject assessment,32 was estimated first using the formula: SEM ⫽ SDT1⫻兹1⫺R, where SDT1 is the SD of the sample at baseline and R is the reliability coefficient. The Cronbach ␣ was used as a reliability measure.33 From the SEM, the MDC was derived as follows32: MDC ⫽ SEM⫻z-score⫻兹2 . A 95% confidence level (MDC95%) was established, corresponding to a z-value of 1.96. The interpretation of MDC95% is that if a patient has a change score at or more than the MDC95% threshold, it is possible to state with 95% confidence that this change is reliable and is not the result of a measurement error. The MCID proportion and the MDC proportion, which are the proportion of the sample with a change scores exceeding the MCID and MDC95%, respectively, were estimated. Finally, the MCID was divided by the MDC95% to determine if the MCID surpassed the MDC95%.34 If this ratio exceeded 1, the MCID can be discriminated from measurement error. All effects were considered statistically significant at P⬍0.05. All statistical analyses were performed using SAS for Windows statistical software, version 8.0 (SAS Inc., Cary, NC). Results During the recruitment period, 7438 cataract removal surgeries were scheduled. Of these, 907 were excluded (377 did not meet the eligibility criteria, 470 were a second intervention performed in the same patient during the recruitment period, and 60 had corneal 419 Ophthalmology Volume 116, Number 3, March 2009 dystrophy). Of the 6531 patients who fulfilled the selection criteria, 5512 (84.40%) agreed to participate and completed the questionnaires before the intervention. Of these, 4356 (79.03%) also completed the questionnaires after surgery. This is the sample included in this study. There were no statistically significant differences between the participants who completed the questionnaires 3 months after surgery and those who did not in age (73.35 vs. 72.72 years, respectively) or gender (58.15% vs. 59.60% were women, respectively). In contrast, nonresponders were more likely to live alone (21.26% vs. 18.21%), to have no education or only to have completed primary studies (87.55% vs. 84.19%), to be widowed or separated (38.02% vs. 30.12%), or to have other ocular pathologies (27.22% vs. 23.40%). The mean preoperative VA and VF-14 results also were lower among nonresponders (0.24 vs. 0.28 and 56.64 vs. 60.99, respectively). After cataract removal surgery, VA increased 0.47 points and VF-14 scores increased 24.03 points. Small but significant increases also were observed in all SF-36 domains, with mean changes ranging from 1.86 points in role emotional to 5.62 points in role physical (Table 1). When evaluated by VA at baseline, the mean changes in VA and VF-14 scores were higher for patients in the lowest VA category at baseline (ⱕ0.1) than for those in the 2 higher categories. The improvements among patients with simple cataract were statistically higher than they were among those with retinopathy or other ocular pathologies. The VF-14 showed minor floor or ceiling effects (⬍3%) before the intervention. The ceiling effect was higher 3 months after surgery (25.69%), whereas the floor effect slightly decreased. The SF-36 domains presented floor and ceiling effects before and after the intervention in 4 domains. Participants who answered “somewhat worse,” “quite worse,” or “a great deal worse” to the transitional question about perceived visual improvement after surgery were combined because of the small sample sizes in each category. The mean changes in VA and VF-14 were significantly different among most of the 5 transitionalrelated categories, with a clear descent from the “a great deal better” to the “worse” category. In the SF-36 domains, the gradient was not as clear as with VA and VF-14, and changes were minor (Table 2). Table 3 shows the different responsiveness parameters for the outcomes measured. Regarding VA, the responsiveness parameters ranged from 1.52 to 2.76, indicating large changes. For VF-14, the parameters ranged from 0.71 to 1.07, also indicating large Table 1. Changes in Visual Acuity 6 Weeks after Intervention and in Health-Related Quality of Life 3 Months after Intervention (N ⫽ 4356) VA VF-14 SF-36 Physical functioning Role physical Bodily pain General health Social functioning Role emotional Vitality Mental health VF-14 by VA at baseline ⱕ0.1 0.2–0.4 ⱖ0.5 VA by VA at baseline ⱕ0.1 0.2–0.4 ⱖ0.5 VF-14 by ocular pathologies Simple cataract Cataract with retinopathy or other pathologies VA by ocular pathologic features Simple cataract Cataract with retinopathy or other pathologies Before Intervention After Intervention Change* P Value† 0.28 (0.17) 60.99 (22.49) 0.75 (0.25) 85.11 (18.10) 0.47 (0.26) 24.03 (24.33) ⬍0.0001 ⬍0.0001 58.19 (27.30) 61.47 (42.88) 61.69 (30.24) 54.03 (20.83) 77.61 (26.08) 79.40 (37.44) 56.23 (23.03) 65.91 (21.17) 61.34 (26.96) 67.05 (41.65) 65.59 (30.27) 56.11 (21.43) 80.52 (24.85) 81.26 (36.11) 59.31 (23.26) 67.99 (21.22) 3.19 (19.18) 5.62 (46.55) 3.66 (26.94) 1.99 (15.71) 2.89 (25.40) 1.86 (38.71) 2.97 (19.74) 2.05 (18.58) ⬍0.0001 ⬍0.0001 ⬍0.0001 ⬍0.0001 ⬍0.0001 0.0026 ⬍0.0001 ⬍0.0001 53.27 (24.85) 62.30 (21.28) 67.37 (20.09) 82.06 (21.98) 85.57 (16.97) 87.85 (15.21) 28.61 (26.90)‡ 23.14 (23.66)‡ 20.57 (21.83)‡ ⬍0.0001 ⬍0.0001 ⬍0.0001 0.07 (0.04) 0.29 (0.09) 0.55 (0.09) 0.64 (0.30) 0.77 (0.22) 0.85 (0.18) 0.57 (0.30)‡ 0.48 (0.23)‡ 0.30 (0.20)‡ ⬍0.0001 ⬍0.0001 ⬍0.0001 62.27 (22.07) 56.94 (23.29) 87.15 (15.91) 78.69 (22.48) 24.77 (23.93)§ 21.70 (25.49)§ ⬍0.0001 ⬍0.0001 0.29 (0.17) 0.24 (0.16) 0.79 (0.22) 0.61 (0.28) 0.50 (0.25)储 0.38 (0.26)储 ⬍0.0001 ⬍0.0001 SF-36 ⫽ Medical Outcomes Short Form 36 Health Survey; VA ⫽ visual acuity; VF-14 ⫽ Visual Function 14 index. Data are given as mean (standard deviation). *The follow-up time for VA was 6 weeks from baseline, for VF-14 and SF-36, 3 months from baseline. Changes were calculated by subtracting postsurgical scores from the baseline scores, with a positive result indicating a gain. † Paired t test for comparison of mean scores between before and after surgery. ‡ P⬍0.0001 for the analysis of variance for the comparison of mean change of VF-14 and VA between subgroups defined by the categories of preintervention VA. § P ⫽ 0.0011 for the Student’s t-test for the comparison of mean change of VF-14 between subgroups defined by ocular pathologies. 储 P⬍0.0001 for the Student’s t-test for the comparison of mean change of VA between subgroups defined by ocular pathologies. 420 Bilbao et al 䡠 Responsiveness, MCID, and MDC after Cataract Surgery Table 2. Changes in Visual Acuity 6 Weeks after Intervention and in Health-Related Quality of Life 3 Months after Intervention According to the Transitional Question Answer to the Transitional Question: “How Is Your Vision Now Compared with How It Was before Your Cataract Surgery?” VA VF-14 SF-36 Physical function Role physical Bodily pain General health Social functioning Role emotional Vitality Mental health A Great Deal Better† (n ⫽ 1378) Quite Better‡ (n ⫽ 1633) Somewhat Better§ (n ⫽ 812) 0.54 (0.23)‡§储¶ 32.46 (23.46)‡§储¶ 0.49 (0.24)†§储¶ 26.50 (21.88)†§储¶ 0.41 (0.25)†‡储¶ 15.57 (20.92)†‡储¶ 4.49 (19.33)§储 10.96 (47.28)‡§¶ 4.57 (25.52) 3.40 (15.76)§储 4.81 (23.85)§ 3.29 (35.43) 5.68 (19.52)‡§储 4.46 (17.68)‡§¶ 3.57 (18.92) 4.44 (44.29)† 4.65 (27.36)§ 2.28 (15.21)§ 2.27 (25.08) 1.67 (37.47) 2.74 (19.19)†§ 1.56 (18.43)† 1.54 (19.20)† –0.12 (47.84)† 0.89 (28.14)‡ 0.12 (15.71)†‡ 0.82 (27.12)† –0.77 (44.63) –0.09 (20.17)†‡ 0.24 (19.12)† Equal储 (n ⫽ 191) 0.28 (0.27)†‡§ 7.71 (21.78)†‡§¶ –0.76 (17.98)† 5.93 (51.19) 0.20 (25.58) –0.73 (15.79)† 5.03 (27.54) 5.62 (41.58) 0.34 (19.98)† 0.78 (19.14) Worse¶ (n ⫽ 173) P Value* 0.27 (0.30)†‡§ –8.12 (24.77)†‡§储 ⬍0.0001 ⬍0.0001 0.82 (19.23) –1.67 (46.44)† 1.86 (27.25) 0.72 (16.85) –1.12 (28.25) –2.47 (42.67) 1.18 (20.49) –2.84 (20.92)† 0.0001 ⬍0.0001 0.0035 ⬍0.0001 0.0007 0.0688 ⬍0.0001 ⬍0.0001 SF-36 ⫽ Medical Outcomes Short Form 36 Health Survey; VA ⫽ visual acuity; VF-14 ⫽ Visual Function 14 index. Data are given as mean (standard deviation). Changes were calculated by subtracting postsurgical scores from the baseline scores, with a positive result indicating a gain. *Analysis of variance for comparison of mean change scores among subgroups defined by the transitional question. †‡§储¶Differences among the 5 subgroups by Scheffé’s test for multiple comparisons at P⬍0.05. The subgroup denoted by Worse combined those patients who answered “somewhat worse,” “quite worse,” or “a great deal worse” to the transitional question because the sample size of those categories was very small. changes, except the Guyatt responsiveness index, which was 0.71, indicating moderate change. The VA and VF-14 responsiveness parameters varied according to VA at baseline, with generally higher responsiveness among those with poorer VA at baseline. Similarly, patients with simple cataract had higher values on those indices than those with retinopathy or other pathologies. Regarding the effect of the better-seeing eye on the results of responsiveness parameters of the VF-14, when the operated eye had better visual acuity than the fellow eye did after surgery, the parameters ranged from 1.06 to 1.19. However, when the fellow eye had better vision than the operated eye did after surgery, the parameters decreased, ranging from 0.76 to 0.82. The different responsiveness parameters of the SF-36 were similar and small for all domains (⬍0.20), indicating no relevant change. The MCID after cataract surgery was 0.41 for VA and 15.57 for VF-14. The MCID was small for SF-36 domains (Table 4). For VA and VF-14, the MCID proportion was near 60%. For SF-36 domains, the MCID proportion was quite variable, ranging from 32.91% to 88.26%. The MDC proportion was 68.59% for the VF-14. For the SF-36, the MDC proportion was similar for all domains, ranging from 6.77% to 19.14%. The ratio of MCID and MDC95% exceeded 1 for the VF-14, but was less than 1 for all SF-36 domains. Discussion The results of this prospective, observational study with a large sample of patients undergoing cataract removal surgery from 17 participating hospitals offer new information about the responsiveness, MCID, and MDC of 3 commonly used measures—VA, the VF-14, and the SF-36. Both VA and the VF-14 were highly responsive, supporting their usefulness for capturing visual changes after cataract removal. In contrast, SF-36 domains were not responsive. Although statistically significant changes were observed in all 3 instruments, clinically significant changes were detected only with VA and VF-14. Impressive mean improvements were observed in VA and VF-14, 0.47 and 24.03 points, respectively, as other investigators have reported.10,27,35 Patients with poorer VA at baseline experienced greater benefit from cataract surgery, as reflected in greater changes in VA and VF-14 scores among patients with poorer VA at baseline, much as has been seen elsewhere.35 In addition, patients with simple cataract, who comprised approximately 75% of the study population, experienced greater gains in VA and VF-14 scores from cataract surgery than those with retinopathy or other ocular pathologies. Interestingly, these improvements were not reflected in SF-36 domains, which showed only minor mean gains. The SF-36 was used as a generic HRQoL measure, and although the changes it measured were statistically significant—mainly because of the large sample size—they were clinically insignificant. For cataract surgery, at least, the SF-36 does not seem to be an appropriate tool for capturing clinically important changes. Floor or ceiling effects indicate the range limits of a HRQoL tool. In this study, VF-14 score did not exhibit either of these effects before the intervention, but did demonstrate a ceiling effect 3 months after surgery. In contrast, the SF-36 presented both floor and ceiling effects before the intervention, implying no room for improvement. Responsiveness parameters, which measure the magnitude of change over time, for VA and VF-14 were substantially above the 0.80 threshold for designating large change.30 These results confirm previous data on the effectiveness of cataract extraction,10,27 although those studies 421 Ophthalmology Volume 116, Number 3, March 2009 Table 3. Responsiveness Parameters 6 Weeks after Intervention in Visual Acuity and 3 Months after Intervention in Health-Related Quality of Life (N ⫽ 4356) VA VA by VA at baseline ⱕ0.1 0.2–0.4 ⱖ0.5 VA by pathologies Simple cataract Cataract with retinopathy or other pathologies VF-14 VF-14 by VA at baseline ⱕ0.1 0.2–0.4 ⱖ0.5 VF-14 by pathologies Simple cataract Cataract with retinopathy or other pathologies SF-36 Physical functioning Role physical Bodily pain General health Social functioning Role emotional Vitality Mental health Standardized Effect Size Standardized Response Mean Guyatt Responsiveness Index 2.76 1.81 1.52 14.25 5.33 3.33 1.90 2.09 1.50 1.74 1.68 0.97 2.94 2.38 2.00 1.46 1.59 1.68 1.07 0.99 0.71 1.15 1.09 1.02 1.06 0.98 0.94 0.90 0.69 0.46 1.12 0.93 1.04 0.85 0.73 0.65 0.12 0.13 0.12 0.10 0.11 0.05 0.13 0.10 0.17 0.12 0.14 0.13 0.11 0.05 0.15 0.11 0.09 –0.002 0.03 0.01 0.03 –0.02 –0.01 0.01 SF-36 ⫽ Medical Outcomes Short Form 36 Health Survey; VA ⫽ visual acuity; VF-14 ⫽ Visual Function 14 index. use only 1 or 2 responsiveness parameters. However, the same results were not observed for the SF-36 domains, another indication that the SF-36 is not an appropriate instrument for capturing clinically important changes in patients with cataracts. As has been seen in previous studies,10,35 a disease-specific instrument, in this case the VF14, was more responsive. As other investigators have pointed out, the VF-14 was more responsive among patients with greater preoperative impairments35 and with simple cataract, where the room for improvement seems to be higher. This finding is reflected in the values of the different responsiveness parameters for VF-14 according to preoperative VA and ocular pathologies. However, something to take into account in the interpretation of the responsiveness of VF-14 is that the global results may be biased toward lower responsiveness indices because of the effect of the betterseeing eye. A clinically important issue is the establishment of relevant individual gains when using HRQoL tools. Statistically significant changes or noteworthy responsiveness parameters at the group level may not be significant at the 422 individual level.31,32 This is where the MCID and MDC play important roles. Based on these results, the MCID for VA is approximately 0.40, whereas that for VF-14 is approximately 16 points. When participants were asked about changes in their visual impairment after cataract extraction surgery, most (71.91%) reported either “a great deal better” or “quite better.” A relatively small group (19.39%) answered “somewhat better.” This somewhat limited the conclusions and the estimation of MCID, because the MCID was based on this category of the transitional question. This highlights a possible limitation of this study. Therefore, when considering these results for the MCID, these limitations must be kept in mind and must be considered with caution because of the uncertainty of these estimators and should not be taken as absolute thresholds.31 The MCID and MDC results differed considerably, largely because of the way each parameter is constructed. The MCID is based on the subjective answer to a transitional question, whereas the MDC is based on the standard error of measurement, which depends on the accuracy and variability of its components. These limitations notwithstanding, the ratio between the MCID and MDC for the VF-14 was higher than 1, indicating that the MCID can be discriminated clearly from measurement error. Nonresponse is a usual finding when conducting follow-up studies.10,35 In this case, there was a very good response rate before the intervention (84.40%) and 3 months after it (79.03%). When comparing those who responded with those who did not, nonresponders were found to have had poorer preintervention VA and VF-14 scores. These might have skewed the results. The 3-month period established for patients to complete the mailed follow-up questionnaires (VF-14 and SF-36) offered enough time to capture visual improvements even after correction with glasses. Although this is similar to the duration of follow-up in some studies,35,36 other investigators used longer follow-up times,10,27 such as 4 months. In conclusion, the results of this prospective, observational study shed some light on the responsiveness of VF-14 for cataract surgery and provide new information about the responsiveness of VA and the SF-36. Although the SF-36 does not seem to be an appropriate tool for capturing clinically important changes after cataract extraction, it is useful to compare data with that from patients with other diseases. Few studies have been conducted on responsiveness parameters of HRQoL-specific instruments for patients undergoing cataract surgery,10,27,35 and there are none, to the best of the authors’ knowledge, about the MCID and MDC of such instruments in this population. The present study provides more complete data on the responsiveness of VF-14 using 3 different responsiveness parameters, and therefore it begins to fill this gap by providing information about the MCID and MDC that should be expected for patients undergoing cataract surgery. This information may help clinicians interpret the effectiveness of cataract extraction in individual patients. However, establishing rigorous, evidence-based Bilbao et al 䡠 Responsiveness, MCID, and MDC after Cataract Surgery Table 4. Minimal Detectable Change and Minimal Clinically Important Difference Parameters 3 Months after Intervention in Health-Related Quality of Life and 6 Weeks after Intervention in Visual Acuity (N ⫽ 4356) VA VF-14 SF-36 Physical functioning Role physical Bodily pain General health Social functioning Role emotional Vitality Mental health Cronbach ␣ Standard Error of Measurement Minimal Detectable Change at the 95% Confidence Level Minimal Minimal Minimal Clinically Clinically Detectable Important Important Change Difference Difference Proportion Proportion Ratio of Minimal Clinically Important Difference to Minimal Detectable Change at the 95% Confidence Level — 0.97 — 3.90 — 10.81 0.41 15.57 — 68.59 57.50 61.72 — 1.44 0.92 0.91 0.88 0.81 0.77 0.92 0.84 0.86 7.72 12.86 10.48 9.08 12.51 10.59 9.21 7.92 21.40 35.65 29.05 25.17 34.68 29.35 25.53 21.95 1.54 ⫺0.12 0.89 0.12 0.82 ⫺0.77 ⫺0.09 0.24 12.33 19.14 15.11 6.77 11.59 14.37 9.68 11.84 43.46 79.38 37.41 43.70 32.91 88.26 67.60 45.05 0.07 ⫺0.003 0.03 0.005 0.02 ⫺0.03 ⫺0.004 0.01 — ⫽ not applicable; SF-36 ⫽ Medical Outcomes Short Form 36 Health Survey; VA ⫽ visual acuity; VF-14 ⫽ Visual Function 14 index. 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Available online: January 24, 2009. 6 Manuscript no. 2008-879. 1 Fundación Vasca de Innovación e Investigación Sanitarias (BIOEF)– CIBER Epidemiología y Salud Pública (CIBERESP), Sondika, Bizkaia, Spain. 2 Unidad de Investigación, Hospital Galdakao–Usansolo–CIBER Epidemiología y Salud Pública (CIBERESP), Galdakao, Bizkaia, Spain. 3 Unidad de Investigación, Hospital de Basurto–CIBER Epidemiología y Salud Pública (CIBERESP), Bilbao, Bizkaia, Spain. 4 Agencia Lain Entralgo, Madrid, Spain. 5 Unidad de Epidemiología Clínica, Corporacio Parc Tauli, Barcelona, Spain. 424 Dirección Territorial de Sanidad, San Sebastián-Donostia, Gipuzkoa, Spain. A complete listing of the members of the IRYSS-Cataract Group is available in Appendix 1 at http://aaojournal.org. Financial Disclosure(s): The author(s) have no proprietary or commercial interest in any materials discussed in this article. Supported in part by the Fondo de Investigación Sanitaria (grant nos.: PI03/0550, PI03/0724, PI03/0471, PI03/0828, PI04/1577); the thematic networks (Red IRYSS) of the Instituto de Salud Carlos III (G03/202), Madrid, Spain; and the Department of Health of the Basque Country (2003/11045), Vitoria, Alava, Spain. Correspondence: Amaia Bilbao, MSc, Fundación Vasca de Innovación e Investigación Sanitarias (BIOEF), Plaza Asua, 1, 48150 Sondika, Vizcaya, Spain. E-mail: abilbao@bioef.org. Bilbao et al 䡠 Responsiveness, MCID, and MDC after Cataract Surgery Appendix 1 The IRYSS-Cataract group included the following coinvestigators: Jesús Martínez-Tapias, Eduardo Aguayo (Hospital Universitario Virgen de las Nieves, Granada); Emilio Perea-Milla (Hospital Costa del Sol-CIBER Epidemiología y Salud Pública [CIBERESP], Málaga); Juan Ramón Lacalle (Facultad de Medicina, Universidad de Sevilla); Eduardo Briones (Hospital Universitario Virgen de Valme, Sevilla); Gemma Navarro (Corporació Sanitaria Parc Taulí, Sabadell); Juan Antonio Blasco, Nerea Fernández de Larrea (Agencia Laín Entralgo, Madrid); Inmaculada Arostegui (Departamento de Matemática Aplicada, UPV-CIBER Epidemiología y Salud Pública [CIBERESP]); Txomin Alberdi (Servicio de Oftalmología, Hospital de Galdakao-Usansolo, Bizkaia); José M. Begiristain (Dirección Territorial de Gipuzkoa); Idoia Garai (Dirección Territorial de Bizkaia); Felipe Aizpuru (Unidad de Investigación del Hospital de TxagorritxuCIBER Epidemiología y Salud Pública [CIBERESP], Alava); Nerea González, Iratxe Lafuente, Urko Aguirre (Unidad de Investigación del Hospital Galdakao-UsansoloCIBER Epidemiología y Salud Pública [CIBERESP], Bizkaia). 424.e1