Design Inception cohort study with one year follow-up.

Setting Primary care clinics in Sydney, Australia.

Participants The study sample was a subcohort of an inception cohort of 973 consecutive patients presenting to primary care with acute low back pain (<2 weeks’ duration). 406 participants whose pain persisted for three months formed the inception cohort of patients with chronic low back pain.

Main outcome measures Outcomes and putative predictors measured at initial presentation, onset of chronicity (study entry), and follow-up at nine and 12 months. Recovery was determined from measures of pain intensity, disability, and work status. The association between potential prognostic factors and time to recovery was modelled with Cox regression.

Results Completeness of follow-up was 97% of total person time for all outcomes. The cumulative probability of being pain-free was 35% at nine months and 42% at 12 months and for complete recovery was 35% at nine months and 41% at 12 months. Of the 259 participants who had not recovered from pain related disability at entry to the chronic study, 47% had recovered by 12 months. Previous sick leave due to low back pain, high disability levels or high pain intensity at onset of chronicity, low levels of education, greater perceived risk of persistent pain, and being born outside Australia were associated with delayed recovery.

The prognosis of chronic low back pain is uncertain. Although clinical guidelines generally portray the condition as having a poor prognosis^{1 4} this is based either on studies of potentially unrepresentative survival cohorts^{5 6} or on studies with large losses to follow-up.^{7 8}

Obtaining a representative cohort is the major problem in designing valid studies on the prognosis of chronic low back pain.⁹ Most studies follow survival cohorts, which sample prevalent cases. These cohorts are likely to over-represent people with longstanding disease and may therefore provide overly pessimistic estimates of prognosis. A more optimal design would be to take a sample from a population at risk of developing chronic low back pain and identify an inception cohort from incident cases.¹⁰

To be eligible to participate in the larger cohort with acute low back pain¹¹ the participants had to be at least 14 years old, provide written consent to participate in the study, speak and read English, and present with pain anywhere in the region of the low back bounded superiorly by T12 and inferiorly by the buttock crease,¹⁴ which had lasted for more than 24 hours but less than two weeks and had been preceded by at least one pain-free month.¹⁵ Participants with pain that had persisted for three months after the onset of symptoms met the criteria for chronic low back pain and were invited to participate in the current study. We excluded patients with a diagnosis of radiculopathy or serious spinal disease, such as cancer, spinal infection, spinal fracture, and inflammatory arthritis. We gave the practitioners a copy of a clinical practice guideline for low back pain¹⁶ and asked them to follow it when appropriate.

Figure 2​2 presents the Kaplan-Meier survival curves for the three outcomes. As the estimated survival did not fall below 50%, 25th centile survival times are presented: 178 days (95% confidence interval 153 to 209) for disability, 192 (170 to 222) for pain, and 197 (176 to 223) for complete recovery. The cumulative probability of being pain-free was 35% (141 events) at nine months and 42% (165 events) at 12 months. Of the 259 participants who had not recovered from pain related disability at entry to the chronic study, 39% (99 events) had recovered by nine months and 47% (118 events) by 12 months. The cumulative probability of complete recovery was 35% (139 events) at nine months and 41% (163 events) at 12 months (fig 2). Only 44 of 406 participants (11%) had not returned to work in their previous capacity at the onset of chronicity and, of these, 46% (20 events) had returned to work in their previous capacity by 12 months.

Table 2​2 shows the characteristics of the study population for pain intensity, pain related disability, and work status at the onset of symptoms (acute presentation), study entry (onset of chronicity), and the follow-up assessments. At the onset of chronicity the cohort had relatively low levels of pain and disability—for example, around two thirds reported pain levels as “mild” or less and disability levels as a “little bit” or less. At nine months, 32% (n=121) of participants reported being free of pain and 59% (n=223) free of pain related disability; respective values at 12 months were 37% (n=142) and 63% (n=238). Before the onset of the acute episode of low back pain, 76% (n=289) of the participants were working full time; this value had decreased to 43% (n=164) at the onset of acute low back pain. At the onset of chronicity, 88% (254/289) of the participants who were employed before the onset of the acute episode had returned to work.

See web extra on bmj.com for the full results of univariate Cox regression analysis. Table 3​3 lists the variables associated with time to recovery from pain. The correlations between the variables feelings of depression and feelings of tension or anxiety and between the variables pain intensity and disability at chronic presentation were greater than 0.6 (Pearson’s r=0.65 for both correlations). Based on ease of assessment and on the univariate hazard ratio, the variables feelings of tension or anxiety and pain intensity were excluded from the multivariate analysis. Of the 15 variables entered into the multivariate model, Cox regression showed that only previous sick leave due to low back pain (likelihood ratio χ²₁=4.9, P=0.03), high disability levels at chronic presentation (χ²₁=20.1, P<0.001), low level of education (χ²₁=3.8, P=0.05), and greater perceived risk of persistent pain (χ²₁=10.0, P=0.002) were significantly associated with delayed recovery. The adjusted hazard ratio for previous sick leave due to low back pain was 0.69 (95% confidence interval 0.50 to 0.97) and for low level of education was 0.74 (0.54 to 1.00). Therefore participants who had taken previous sick leave due to low back pain and those with no education beyond secondary school were 31% and 26%, respectively, less likely to recover from pain at any time in the future compared with those without these characteristics. The adjusted hazard ratio for disability level was 0.68 (0.56 to 0.81) and for perceived risk of persistent pain was 0.91 (0.86 to 0.97). As these variables are continuous the interpretation is that for every unit increase in these scales, participants are 32% and 9% less likely, respectively, to recover from pain at some time in the future.

Table 4​4 lists the variables associated with time to recovery from disability. Owing to the high correlation between the variables feelings of tension or anxiety and depression (Pearson’s r=0.66) and between pain intensity and disability (Pearson’s r=0.65) these were excluded from the multivariate model. Of the 15 variables entered into the multivariate model, Cox regression showed that only high levels of disability at chronic presentation (likelihood ratio χ²₁=13.5, P=0.0003), greater perceived risk of persistent pain (χ²₁=17.0, P<0.0001), and being born outside Australia (χ²₁=10.7, P=0.001) were significantly associated with delayed recovery as a result of disability. The adjusted hazard ratio for participants born outside Australia was 0.51 (95% confidence interval 0.33 to 0.78). Therefore participants who were born outside Australia were 49% less likely at some time in the future to recover from disability than those who were born in Australia. The adjusted hazard ratio for disability level was 0.69 (0.57 to 0.85) and for perceived risk of persistent pain was 0.88 (0.82 to 0.94). As disability level and perceived risk of persistent pain are continuous variables, for every unit increase in these scales participants were 31% and 12% less likely, respectively, to recover from disability at some time in the future.

Cite this as: BMJ 2009;339:b3829