When is a medical device substantially equivalent to another device? Like so much else, it depends. On July 15, 2014, the U.S. Food and Drug Administration (FDA) issued a draft guidance that aims to clarify that question. The document is officially directed at FDA staff, but it is equally instructive to companies seeking FDA approval for their devices. 

The FDA reviews medical devices before they can be sold in the United States, and manufacturers that prove their devices are substantially equivalent to existing products can undergo an abbreviated review process. However, substantial equivalence is a subjective target.  

The U.S. Food, Drug and Cosmetics Act at 21 U.S. Code 360c(i) states that a new device is substantially equivalent to an existing "predicate" device when the new device:

1. has the same intended use as the predicate device; and
2. either has the same technological characteristics as the predicate or has 
   different technological characteristics but is as safe and effective and does not
   raise different questions of safety and effectiveness than the predicate.

The guidance document focuses on the last step of the analysis – how FDA determines a device is as safe and effective as a predicate. The safety and effectiveness need not be identical. A new device can have increased safety and decreased effectiveness – or decreased safety and increased effectiveness – and still be considered substantially equivalent. When making these assessments, the FDA will weigh the benefits and risks of the new device versus the predicate. When considering benefits, the FDA will weigh:

• Type of benefit
• Magnitude of the benefit
• Probability of the patient experiencing the benefit
• Duration of the benefit

When assessing risks, the FDA will consider:

• Severity, types, number, and rates of harmful events associated with use of the device
• Probability of a harmful event
• Probability of a patient experiencing one or more harmful events
• Duration of harmful events
• Risk from false-positive or false-negative results (for diagnostic devices)

The guidance provides several examples, including one in which a manufacturer wishes to market a tool for spinal surgery. The tool has a different shape and a deeper cutting action than the predicate device. Animal and clinical studies show the deeper cutting action leads to a greater risk of injuring surrounding tissue, but the new tool also shortens the duration of surgery and allows improved access to certain parts of the anatomy. The guidance concludes that the new device does not raise different questions of safety and effectiveness, and because the increased risk is accompanied by an increased benefit and a comparable benefit-risk profile, the new device would likely be found substantially equivalent to the predicate. 

Of course, the specific analysis will vary for every product and predicate. Companies seeking FDA approval should consult the guidance and an attorney to help prove their devices are substantially equivalent.