As we recently highlighted (see also here and here and M. Catley’s post) tactile acuity assessment has become an increasingly popular area of research in chronic pain. Most two point discrimination (TPD) studies focus on chronic low back pain, but TPD has also been investigated in other conditions that are challenging to treat, e.g. complex regional pain syndrome or phantom limb pain. TPD is thought to reflect cortical changes associated with chronic pain. If this is the case, novel treatments such as sensory discrimination training to normalize sensory function might have the potential to affect such changes and maybe improve pain. The body of evidence suggests that chronic pain is associated with reduced tactile acuity, disrupted body-image, and impaired body awareness.
The question is why is TPD becoming so popular? Perhaps because a two-point discrimination test seems to be simple, quick and easy to perform. No expensive equipment is required for this test, only a caliper (e.g. mechanical) that can be purchased in any home store. It is tempting to draw “simple” conclusions from this “simple” examination. But is it just as “simple” as it seems? What about the reliability of the test? We believe that not only the caliper is required, but also specific training
So how to assess tactile acuity reliably? Is it possible to measure it reliably at all? In the last issue of Musculoskeletal Science & Practice journal these questions were raised in the systematic review of Ehrenbrusthoff et al. . We read it carefully and decided to initiate a debate around the reliability and standardization of two-point discrimination assessments . Ideally, tactile acuity assessment should be standardized for clinical and research purposes as accurately as the well-established Quantitative Sensory Testing Procedure . Standardization improves data reporting and its comparability. We support our viewpoint with the following two suggestions:
- TPD has the potential to be a reliable assessment technique across different body parts. We emphasized in our letter, that the reliability of TPD in the most prevalent condition, i.e. low back pain, and in healthy controls is sufficient ; studies reported intra-rater reliability as moderate to excellent (ICC=0.50–0.90) and inter-rater as moderate to good (ICC=0.56–0.76). In other pain conditions, reliability was also reported to be sufficient. That is not to say that the reliability cannot be improved by standardizing the way it is tested.
- Carefully designed standardization based on all of the available evidence is a crucial step for the future development of the field. Currently, TPD is measured with a wide range of protocols for different body regions, and large variabilities within the same body locations have been reported (see  Table 7 for summary of protocols for LBP: ~16/19 protocols are different!). Guidelines for a standardized protocol for TPD testing need to be developed by expert consensus for all body regions. Clinically, two-point discrimination seems to be a proxy for central nervous system function and is useful for pain profiling and individualized management.
Regardless of whether you agree or disagree with our post we encourage you to share your opinion about tactile acuity and TPD standardization. Any feedback is useful! email@example.com.
About the authors
Wacław Adamczyk is a physiotherapist, PhD fellow at the Jerzy Kukuczka Academy of Physical Education in Katowice, Poland. His research focuses on perceptual distortions in acute and chronic pain and the role of learning processes in the formation of analgesic and hyperalgesic effects. Except muscles soreness induced by running he loves mountains, pizzas and strong coffees.
Kerstin Luedtke is a Professor for Physiotherapy at the University of Luebeck. Her clinical background in the multidisciplinary management of patients with chronic spinal pain influences her research interests which is centered around central nervous system alterations in chronic pain. A second line of research focuses on the influence of cervical dysfunctions to primary headaches.
Tibor Szikszay is a physiotherapist and is currently doing his PhD at the University of Lübeck in Germany. His research focuses chronic pain and endogenous pain modulation. He knows, just as you can fight fire with fire, you can affect pain with pain, regardless of whether it makes sense or not.
 Ehrenbrusthoff K, Ryan CG, Grüneberg C, Martin DJ. A systematic review and meta-analysis of the reliability and validity of sensorimotor measurement instruments in people with chronic low back pain. Musculoskelet Sci Pract. 2018 Jun; 35:73-83. doi: 10.1016/j.msksp.2018.02.007. Epub 2018 Mar 2. Review. PubMed PMID: 29549815.
 Adamczyk WM, Luedtke K, Szikszay TM. Two-point discrimination and low back pain: Not as unreliable as it seems, but what about standardised procedures? Musculoskelet Sci Pract. 2018 Jun;35:e110-e111. doi: 10.1016/j.msksp.2018.03.007. Epub 2018 Mar 27. PubMed PMID: 29606530.
 Rolke R, Baron R, Maier C, Tölle TR, Treede RD, Beyer A, Binder A, Birbaumer N, Birklein F, Bötefür IC, Braune S, Flor H, Huge V, Klug R, Landwehrmeyer GB, Magerl W, Maihöfner C, Rolko C, Schaub C, Scherens A, Sprenger T, Valet M, Wasserka B. Quantitative sensory testing in the German Research Network on Neuropathic Pain (DFNS): standardized protocol and reference values. Pain. 2006 Aug;123(3):231-43. Epub 2006 May 11. Erratum in: Pain. 2006 Nov;125(1-2):197. PubMed PMID: 16697110.
 Adamczyk W, Luedtke K, Saulicz E. Lumbar Tactile Acuity in Patients With Low Back Pain and Healthy Controls: Systematic Review and Meta-Analysis. Clin J Pain. 2018 Jan;34(1):82-94. doi: 10.1097/AJP.0000000000000499. PubMed PMID: 28328700.