Table 1 Study characteristics

Gebhardt et al., 2006 [23]

Prospective cohort

NSLBP group received 3 weeks of daily treatment during clinical period (including oral pain medication and physical therapy).

NSLBP group data collected on days 0, 3, 7, 10, 14, 17 and 21 (clinical period) and after 2, 3 and 6 months (follow-up).

41 participants with NSLBP; 1572 controls representative of German population

NSLBP defined as over 3 months of chronic myofascial LBP with absence of radicular symptoms or motor deficits.

Heffner et al., 2011 [24]

Case-control

Chronic NSLBP group compared to healthy control group of age and sex-matched individuals over 24 h period.

Blood samples taken and questionnaires completed in morning of Day 1 (time 1). Sleep quality and pain recorded morning of Day 2 (time 2).

25 participants with chronic NSLBP;

25 age- and sex-matched controls without pain.

Chronic NSLBP defined as more than 6 months pain duration without history of inflammatory disease, spine surgery, orthopaedic injury or neurologic signs.

Klyne et al., 2017^a [25]

Case-control

Participants were divided by: (1) those with and without NSLBP, and (2)“high pain,” those with moderate-to-severe NSLBP (VAS ≥4), “low pain,” those with mild LBP (VAS < 4), and controls.

Blood samples collected at one time point and questionnaires completed with 24 h.

99 participants with acute NSLBP;

55 healthy controls

Acute NSLBP was defined as an episode within < 2 weeks prior following at least 1 month with no pain. Episodes lasted > 24 h and caused functional limitation and care seeking.

Klyne et al., 2018^a [26]

Prospective cohort

Participants were categorized into NSLBP and control groups. NSLBP participants were then categorized based on their past 6 months of pain and disability status.

Blood samples collected at baseline and 6 months. Pain questionnaires completed within 24 h of sampling and every 2 weeks for 6 months.

Demographics are reported above for Klyne 2017 study.

NSLBP participants were categorized in 3 groups based on their NRS pain scores and RMDQ scores at 6 months: (1) unrecovered - increased/unchanged pain and disability from baseline, or a pain score ≥ 7/10 (unrecovered) (2) partially recovered - pain and/or disability is decreased but not yet fully recovered from baseline, and (3) recovered - no pain and disability. NSLBP participants (N = 25) without these data were not categorized.

Luchting et al., 2016 [27]

Case-control

NSLBP group compared to neuropathic pain group and healthy control group

Blood samples taken once between 9:00 and 9:30 AM. Self-reported pain, stress and depression questionnaires administered.

19 participants with chronic NSLBP; 19 participants suffering from neuropathic pain;

19 subjects that are pain-free.

NSLBP defined as persistent low back pain not attributable to a detectable pathology

Sturmer et al., 2005 [28]

Prospective cohort

NSLBP group compared to acute sciatic pain group for duration of 6 months.

Blood samples collected and outcomes assessed on day 3, 7, 10, 14, 17, 21 and after 2, 3, and 6 months.

41 participants with chronic NSLBP (65.8% female, mean age 42.4 years, mean BMI 27.7 kg/m² SD 6.8, 41.5% current smokers)

NSLBP defined as LBP for at least 6 weeks attributable to the spine and without signs of specific pathology.

Wang et al., 2010 [29]

Prospective Cross-sectional study

NSLBP group was compared to a NSLBP with depression group, and healthy age and sex-matched control group at one time point.

Blood samples collected and outcomes assessed at one time point.

29 participants with chronic NSLBP; 29 patients with NSLBP and depression; 29 age and sex matched healthy controls.

NSLBP defined as LBP of at least 6 months duration in absence of specific etiology. Leave from work for at least 6 weeks.

Wang et al., 2008 [30]

Case-control

Matched pair design for a 6 month duration. NSLBP group underwent 3 weeks of inpatient biopsychosocial and physical therapy.

Blood samples collected and outcomes assessed at Day 0, 10, 21 and 6 months.

120 participants with chronic NSLBP; 120 age and sex matched controls with no LBP in the past year (43.3% female, mean age 45.4 SD 11.4 years, mean BMI 27.1 kg/m² (18.7–47.8), 31.7% current smokers)

NSLBP defined as unspecific myofascial chronic LBP, present for a least 3 months. Subjects with specific causes of pain or other locations of pain were excluded. Leave from work for at least 6 weeks.