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A letter to the editor is presented in response to the article "Cervical Medial Branch Blocks for Chronic Cervical Facet Joint Pain: A Randomized, Double-Blind, Controlled Trial With One-Year Follow-Up," by L. Manchikanti, V. Singh and F. J. Falco in the previous issue of the journal.

Study Design: Retrospective study of pressure-controlled discography (PCD) in patients with chronic low back pain (CLBP)., Objectives: Define the distribution of disc pain thresholds and determine, using an analytical model, if a false-positive subgroup could be identified., Summary of Background Data: The characteristics of the distribution of disc pain thresholds may allow identification of subgroups within a population., Methods: Disc pain thresholds were determined using PCD in patients with CLBP referred for discography. An analytical model and previously published data from an asymptomatic group were used to determine the distribution of false-positive discs., Results: A total of 838 discs from 253 patients were studied. Regression analysis demonstrated no significant association between levels from patients with CLBP with more than one positive disc. The distribution of discography pain thresholds from the patients with CLBP revealed two distinct groups: contact-sensitive (CS) and pressure-sensitive (PS). Discs in the CS group have a pain threshold of 0 psi, indicating that contact between the contrast and the disc provokes pain. Discs in the PS group have a pain threshold of > 0 psi, demonstrating that internal pressure must be generated to provoke pain. PS discs consist of moderately and minimally sensitive subgroups, with overlapping distributions. The minimally sensitive subgroup corresponds to false-positive discs. At 50 psi, the probability of a false-positive disc is 100%, at 25 psi 50%, at 19 psi 25%, and at 14 psi 10%., Conclusions: Discography should be performed under pressure control, as doing so allows the pain threshold of discs to be quantified, and the probability that a disc is false-positive estimated.