605 patients fulfilled eligibility criteria and were interviewed at baseline. This represents 25% of the 2,454 respondents to invitations mailed to 42,650 patients who were seen for any kind of LBP in clinics of the HMO during the twelve months of recruitment. 521 patients (86%) completed a 6-month follow-up interview. shows mean self-ratings on the GPE scale and mean pain and disability scores for six response levels. At 6-month follow-up, 32% of patients reported to be “fully recovered”, 81% to be at least “much improved” and 91% at least “slightly improved”. If we classify patients reporting to be “slightly improved” as recovered (Reference Standard 1), 47 (9%) would be classified as non-recovered; if we classified the same patients as non-recovered (Reference Standard 2), we would classify 98 (19%) of all patients as non-recovered.
Mean Values and Standard Deviation (SD) for Pain and Disability at Follow-up as well as percent change from baseline by Level of Global Perceived Effect
shows the average percentage changes in pain and disability from baseline to 6-month follow-up for each GPE score. For the “completely recovered” and “much improved” GPE groups, pain and RM disability similarly improved on average by approximately 100% and 80%. The two GPE groups reported “slightly improved” or ”same” improved by 30–40% with mean RM change scores being identical for both groups (41%). The finding that patients with 30–40% improvement in pain or disability may report their follow-up situation as being the “same” illustrates the potential for misclassification if we use a single criterion of GPE, pain or disability for discriminating between recovery and chronic pain.
Percent Change from Baseline (95% Confidence Intervals (CI)) by Level of Global Perceived Effect (GPE)
shows the proportions of patients in each GPE category who improved by more than 50% or 30%, respectively, from baseline to six months. Though the proportions of patients who improved by either 30 or 50% in a parameter were quite similar within the subgroups at both ends of the GPE scale (“much improved” and “much worse”), these proportions clearly differed in the GPE scale's middle range. In “slightly improved” patients, less than half of the patients reported a 50% reduction in pain or disability; in this GPE group the mean RM score was 8 (median = 7; see ) which is above the reference standard score of ≥7 for chronic pain in several prior studies.9, 36
Consequently, half of these patients would fall into the chronic pain outcome group if we used a RM score of ≥7 or a 50% reduction in pain and function as reference. These findings question the accuracy of a dichotomous outcome using the GPE scale and classifying “slightly improved” patients as recovered2
. Although the number of self-reported “slightly worse” patients in our sample is too small (N
= 9) to draw general conclusions, choosing a 30% improvement in the RM score as criterion for improvement would classify more than half of these as improved and therefore render this choice problematic. To reiterate, in general dichotomous classifications based on a single criterion may be problematic.
Proportion of patients for each level of GPE which have improved in a parameter by 50 / 30%
In which way do “completely recovered” patients differ from “much improved” patients? Almost half of the patients (117 of 253; 46%) reportedly not “completely recovered” but “much improved” were free of pain at 6 months, with a mean RM score of 1.8 (SD ± 2.5) (data not presented in tables). In other words, the majority of “much improved” patients reported pain in the past week rated 1.8 for average intensity and 4.2 for worst pain. Generally, if at follow-up patients still had pain, worst pain in the last week was considerably higher than average pain (“slightly improved”: 5.6 vs. 3.3; “same”: 7.0 vs. 4.6). Worst pain in the past week, in addition to average pain intensity, may be a key aspect of GPE self-classification.
to show reference standard-based cut-offs (and areas under the corresponding ROC curves) for: MIC values for pain and disability (), upper limits of pain and disability still compatible with self-reported recovery (), and minimally important percent changes for pain and disability from baseline (). Absolute values for MIC in pain or disability scores were expected to vary according to baseline scores; therefore we present separate results for patient subgroups with baseline scores either above () or below () the median. shows confidence intervals estimated by bootstrapping to the results of .
Minimal Important Change from Baseline to Perceived Recovery (AUC*; N = 521) for
Minimum Percent Change from Baseline to Perceived Recovery (AUC*) for
Upper Limits for Pain and Disability Compatible with Perceived Recovery (AUC*) for
Same as but only in patients with above median baseline scores
Same as but only in patients with below median baseline scores
Minimum Percent Change from Baseline to Perceived Recovery (Confidence Intervals by bootstrapping) for
Each table presents five rows of data for five different reference standards. For easy comparison, all reference standards are listed in a single legend in to . As de Vet et al. suggested2
, with Reference Standard 1 patients were counted as recovered, if they were “fully recovered”, “much improved” or “slightly improved”, whereas with Reference Standard 2 “slightly improved“ patients were counted as non-recovered. Reference Standards 3, 4 and 5 add conditions to the patients self-classified as “slightly improved” or “same”. These patients were counted as recovered if they had pain of less than 3 out of 10 (NRS; Reference Standard 3), disability of less than 4 out of 24 on RM scale (Reference Standard 4) or fulfilled both conditions (Reference Standard 5). These cut-offs were taken from the assessment of the upper limits of these values for compatibility with self-reported recovery according to Reference Standard 1.
Using Reference Standards 3, 4 or 5 with combined criteria, 70 (13%), 82 (16%) or 67 (13%) patients, respectively, would be classified as having chronic LBP. In our sample of patients with acute LBP, perceived recovery required percent changes from baseline pain and disability to be well above 50%. As expected, absolute values for MICs were dramatically higher for patients with higher baseline scores than for those with lower baseline values.
In addition to average pain in the past week, we assessed bothersomeness of pain, a parameter used in numerous previous LBP studies5, 20, 30, 36–39
. All of our analyses showed virtually identical results for both pain measures (data not presented). Regarding the parameter's ability to discriminate between recovery and non-recovery, bothersomeness of pain in the past week was not superior to average pain in the past week (p
-values for comparing AUCs were between 0.12 and 0.76). As expected, integrating pain or disability or both into the classification criteria for recovery or non-recovery improves the discriminative ability. Among the combination criteria, the discriminatory accuracy appears to be strongest with the inclusion of either pain into the GPE scale, or both pain and disability conjoined.