Two devices are said to be equivalent, for a particular measurement technology, if it is proven that all of the hardware and firmware components used in the determination of that measurement are identical for both devices. Components may differ, provided that they are also proven to be equivalent.
The most complete guidelines on how to prove equivalence were drawn up by the Health Technology and Cosmetics department of the European Commission . In simple terms, this involves devising a checklist of everything involved in a device used to measure some medical value. This list is split into two sets, all the items directly involved in the measurement and all of the other items. The list can be filled in for any device and, once this is done, those devices can be compared.
If all of the items directly involved in the measurement match each other then, as far as taking that measurement is concerned, the devices are said to be equivalent and it doesn't matter which device is used. All devices that are proven to be equivalent to each other form a "family" of devices. Equivalence has major consequences because it means that the results, good or bad, of any validation of the measurement technology must be applied to all equivalent devices. It must be proven and peer-reviewed in the same way as any scientific procedure [2, 3].
The comparison does not just end at equivalence – it has another important function in identifying how any pair of equivalent devices actually differ. They are, after all, being sold as being different. This is found by comparing "all the other items". Sometimes one device has a few extra features than another device, like more memory or more analysis of the results. Sometimes they differ in use, like portable and table-top versions. Sometimes the only difference is in the branding but this too is important to prove, as it protects against fake copycat designs.
Devices that are listed as being derivative of other models have been approved for addition to these lists on the basis of information supplied to the BIHS by the manufacturer. This information has not been rigorously tested but has been accepted in good faith.
The term "derivative" is used by some societies as a synonym for equivalence. The BIHS accepts declarations, in good faith, on information supplied on, a short form, by manufacturers [4, 5]. Hypertension Canada  require sworn affidavits. The DHL approve devices that are technically identical to another already validated device ("technisch baugleich mit einem bereits validierten anderen Messgerät") but information on how that is determined and on which validated devices are involved is not disclosed . The ESH accept both BIHS and DHL listings but not those of Hypertension Canada .
From a scientific perspective, the declaration, is just the first step in the process [2, 3]. This is no different, in principle, to any validation. No manufacturer is going to send a device for validation unless they believe it to be accurate and have likely tested it thoroughly already in their own internal quality control procedure, often in a dry run of the validation. Indeed, these internal checks are often accepted by regulatory authorities. The problem, of course, is that there are also fake claims by unscrupulous companies – which are often impossible to trace. Genuine errors can also be made. This is why the scientific community requires independent peer-reviewed proof [2, 3] and why the EU has moved to require it also from mid 2020 . The World Hypertension League has called on the private sector and on governmental organisations to ensure that the only automated BP devices that are and that can be sold are those that have been tested independently .
This is why Medaval puts a question watermark (Query-mark) over devices that have not been proven scientifically to be accurate. From a scientific perspective, based on published recommendations, we cannot accept that two devices are equivalent just on the say-so of the manufacturer. However, we recognise where the first step in the process has been taken, and accepted as the complete process by some societies, so we have created an assessment category named "Equivalence awaiting proof".
Medaval has also drawn up, with the assistance of renowned experts, a protocol for comparative-equivalence (emphasising the two important aspects of the checks) in compliance with MEDDEV 2.7/1 rev 4  and EU 2017/745 . Every feature is compared [2, 3] – for instance there are 306 items to be compared for blood pressure monitors at present (the list increases to incorporate new innovations). A full analysis of the two sets of items, with each being divided into three subsets, is carried out and an extensive report is produced for provision to a notary body (for regulation purposes) and a summary report, without sensitive proprietary details, for publication. These are peer reviewed [2, 3].
Validation is not included in the comparative-equivalence procedure, as this is dependent only on the device hardware and firmware. However, once equivalence is proven, a short report is written applying any existing validations to the technology to all devices sharing that technology. Similarly, all subsequent validation information is applied to all devices sharing the technology.
From January 2020, Medaval only accepts equivalences to MDR standards [1, 9]. During this period of transition to MDR standards, devices with declared equivalence/derivative to another device, with published evidence of validation, are indicated as having possible MDR equivalence and are not shown with a Query-mark.
- European Commission – Health Technology and Cosmetics. MEDDEV 2.7/1 rev.4: Guidelines on Medical Devices – Clinical Evaluation: A Guide for Manufacturers and Notified Bodies under Directives 93/42/EEC and 90/385/EEC. Brussels, Belgium: European Commission; June 2016 (65 p). Available from:http://ec.europa.eu/DocsRoom/documents/17522/attachments/1/translations/
- O'Brien E, Stergiou GS. The pursuit of accurate blood pressure measurement: A 35-year travail.J Clin Hypertens (Greenwich).2017 Aug;19(8):746-752. doi:10.1111/jch.13005. Epub 2017 Apr 21.PMID: 28429879.
- O'Brien E, Stergiou GS, Turner MJ. The quest for accuracy of blood pressure measuring devices. J Clin Hypertens (Greenwich). 2018 Jul;20(7):1092-1095. doi: 10.1111/jch.13279. PMID: 30003703.
- British and Irish Hypertension Society. BP monitors, Derivative Devices. [Internet] Leicester, UK: BIHS; 2017/2018.https://bihsoc.org/bp-monitors/.
- British and Irish Hypertension Society. Process for Listing of Validated Blood Pressure Monitors. [Internet] Leicester, UK: BIHS; 2017/2018.https://bihsoc.org/bp-monitors/process-for-listing-of-validated-blood-pressure-monitors/.
- Hypertension Canada. Blood Pressure Devices Recommended by Hypertension Canada. [Internet] Markham, Ontario, Canada: Hypertension Canada; 2018.https://hypertension.ca/hypertension-and-you/managing-hypertension/measuring-blood-pressure/devices/.
- Deutsche Hochdruckliga e.V. DHL [German Hypertension League]. Geprüfte Messgeräte 2001-2018 [Tested measuring instruments 2001-2018]. [Internet] Heidelberg, Germany: DHL; 2002-2018.https://www.hochdruckliga.de/messgeraete-mit-pruefsiegel.html.
- Stergiou GS, Asmar R, Myers M, Palatini P, Parati G, Shennan A, Wang J, O'Brien E; European Society of Hypertension Working Group on Blood Pressure Monitoring and Cardiovascular Variability. Improving the accuracy of blood pressure measurement: the influence of the European Society of Hypertension International Protocol (ESH-IP) for the validation of blood pressure measuring devices and future perspectives. <i>J Hypertens</i>. 2018 Mar;<b>36</b>(3):479-487. doi: 10.1097/HJH.0000000000001635. PMID: 29384984
- The European Parliament and the Council of the European Union. Regulation (EU) 2017/745 of the European Parliament and of the Council of 5 April 2017 on medical devices, amending Directive 2001/83/EC, Regulation (EC) No 178/2002 and Regulation (EC) No 1223/2009 and repealing Council Directives 90/385/EEC and 93/42/EEC (Text with EEA relevance).Official Journal of the European Union.2017 May 05;60(L 117):1-175. Available from:https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A32017R0745.
- Campbell NR, Gelfer M, Stergiou GS, Alpert BS, Myers MG, Rakotz MK, Padwal R, Schutte AE, O'Brien E, Lackland DT, Niebylski ML, Nilsson PM, Redburn KA, Zhang XH, Burrell L, Horiuchi M, Poulter NR, Prabhakaran D, Ramirez AJ, Schiffrin EL, Touyz RM, Wang JG, Weber MA. A Call to Regulate Manufacture and Marketing of Blood Pressure Devices and Cuffs: A Position Statement From the World Hypertension League, International Society of Hypertension and Supporting Hypertension Organizations.J Clin Hypertens (Greenwich). 2016 May;18(5):378-80. doi: 10.1111/jch.12782. Epub 2016 Feb 8.PMID: 26852890.