在IVDR法规中，伴随诊断产品的定义如下【Article 2，Definitions，(7)】：
‘companion diagnostic’ means a device which is essential for the safe and effective use of a corresponding medicinal product to:
“伴随诊断产品”指的是对于相应药品的安全和有效使用必不可少的器械，用以：
(a) identify, before and/or during treatment, patients who are most likely to benefit from the corresponding medicinal product;
在治疗之前和/或治疗期间，识别最有可能受益于相应药品的患者；
or 或者
(b) identify, before and/or during treatment, patients likely to be at increased risk of serious adverse reactions as a result of treatment with the corresponding medicinal product;
在治疗之前和/或治疗期间，识别可能由于使用相应药品治疗而导致严重不良反应风险增加的患者；
在IVDR法规立法背景第（11）条指出：
Companion diagnostics are essential for defining patients' eligibility for specific treatment with a medicinal product through the quantitative or qualitative determination of specific markers identifying subjects at a higher risk of developing an adverse reaction
to the medicinal product in question or identifying patients in the population for whom the therapeutic product has been adequately studied, and found safe and effective. Such biomarker
or biomarkers can be present in healthy subjects and/or in patients.
伴随诊断产品对于通过定量或定性确定特定标志物来确定患者是否有资格接受药物的特定治疗至关重要，该标志物用于识别对所述药物产生不良反应的风险较高的受试者，或用于识别人群中治疗产品已被充分研究并发现安全有效的患者。这样的一种或多种生物标志物可以存在于健康受试者和/或患者中。
立法背景第（12）条指出：
(12) Devices that are used with a view to monitoring treatment with a medicinal product in order to ensure that the concentration of relevant substances in the human body is within the therapeutic window are not considered to be companion diagnostics.
用于监测药物治疗以确保人体内相关物质浓度在治疗窗口内的器械，不被视为伴随诊断产品。
但是在IVDR法规中，并没有找到伴随诊断产品的相关举例。
FDA体外伴随诊断产品指南中，对体外伴随诊断产品的定义如下：
An IVD companion diagnostic device could be essential for the safe and effective use of a corresponding therapeutic product to:
Identify patients who are most likely to benefit from the therapeutic product
Identify patients likely to be at increased risk for serious adverse reactions as a result of treatment with the therapeutic product
Monitor response to treatment with the therapeutic product for the purpose of adjusting treatment (e.g., schedule, dose, discontinuation) to achieve improved safety or effectiveness
Identify patients in the population for whom the therapeutic product has been adequately studied, and found safe and effective, i.e., there is insufficient information about the safety and effectiveness of the therapeutic product in any other population
（这一条虽不被IVDR法规中的伴随诊断产品定义所覆盖，但是与IVDR法规立法背景中第（11）条的描述一致）
与IVDR法规中不同的是，FDA将“监测对治疗产品治疗的反应，以调整治疗（如计划、剂量、停药），从而提高安全性或有效性”的器械也作为体外伴随诊断产品。看上去与IVDR立法背景第（12）条的要求相悖。
那么我们来看一下FDA体外伴随诊断产品指南中关于伴随诊断产品的一个示例：
HER-2测试，用于确定患者是否可以接受赫赛汀（曲妥珠单抗）治疗，该药物适用于治疗转移性乳腺癌和胃癌。赫赛汀在HER-2标志物阴性人群中缺乏有效性，也有可能引起严重的不良反应。
目前FDA cleared或批准的伴随诊断产品有157个：
最近的一个是Abbott RealTime IDH1，它是一种体外聚合酶链式反应（PCR）检测方法，用于定性检测从人类血液或骨髓提取的DNA中编码五个IDH1 R132突变（R132C、R132H、R132G、R132S和R132L）的单核苷酸变体（SNV）。Abbott RealTime IDH1被认为有助于识别异柠檬酸脱氢酶-1（IDH1）突变的急性髓细胞白血病（AML）患者，用于TIBSOVO®（Ivosidenib）治疗。
部分截图如下：
