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What is a "diagnostic" block?

Fred on August 13, 2019 at 8:09 pm
  • Author
    • Fred BagaresFred
      Joined: Aug 3, 2017
      Posts: 0

      How is “diagnostic” defined?

      I consider the double block paradigm to be the standard protocol for diagnostics of facet mediated pain. I practice in the US and 80% relief has been my “threshold” for diagnostic. Using this threshold it seems to be fairly predictive for response to RF. It seems to be widely excepted by my peers and 3rd party payors.

      However let me describe these scenarios:
      #1 post-IA knee joint injection with lidocaine a patient’s pain decreases from 8 to 4
      #2 post IA knee joint injection with lidocaine a patient’s pain decreases 4 to 2
      #3 post IA knee joint injection with lidocaine a patient’s pain decreases 2 to 1

      All of these patients have 50% improvement but I’m most “impressed” with scenario #1 compared to #3. I’ve had some patients say that a pain decrease from 4 to 2 was 90% better.

      When I think about this it makes me think about what is truly “diagnostic?” Are all physicians “convinced” of all scenarios?

      Is there research or literature that looks on physician bias when interpreting responses to diagnostic blocks?

      • Jan willem
        Joined: May 7, 2019
        Posts: 3

        different answers are possible

        1: should you treat this pain ( any pain) when Vas scores are below 5?

        2: if you perform a test block; ow do you define succes: only with vas scores, or also with objective measures? is 50% enough; is 80% better; should we perform 1 testblock or a double block?

        3: in this specific case to further open the discussion: recently a publication showed no added value to a test block in this specific knee pain

        Original Research Article
        A Prospective Randomized Trial of Prognostic
        Genicular Nerve Blocks to Determine the
        Predictive Value for the Outcome of Cooled
        Radiofrequency Ablation for Chronic Knee Pain
        Due to Osteoarthritis

        • Robert RapcanRobert Rapcan
          Joined: Dec 6, 2018
          Posts: 7

          Bogduk defined specific criteria for an optimal selection as an anatomically accurate block under guidance with ideally complete relief of pain following an MBDR block. Manchikanti et al. defined at least an 80% reduction of pain and the ability to perform previously painful movements . More liberal criteria have also been reported, such as greater than 50% relief of pain.

          1, 4 to 2 or 2 to 1: It doesnˇt make sense to indicate those patients for diagnostic blocks.

          2,The amount of relief low back pain patients get from facet joint injections can be predicted by psychological factors. Why would that be? Numbing might provide strong enough temporary relief to create an overconfident “eureka!” And that confidence might itself deliver some pain relief, further clouding the issue, and adding up to a false positive: a misleading result that puts a spotlight on the facet joint that is actually innocent, or only part of the problem. …read more at:

          • Fred BagaresFred
            Joined: Aug 3, 2017
            Posts: 0

            Thank you for the responses. While I understand that not every pain needs to be treated, I’m really more interested in the diagnostic part of things and how we arrive at our conclusions. Are blocks less reliable or valid if the VAS is below a certain level? If its a 1 or a 10, theoretically I don’t think it should matter.


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