有段医学英语不懂求助

翻译中碰到一段话，请大神指导：

One major difficulty is that the multidisciplinary approach, by definition, integrates all available diagnostic information and therefore no independent diagnostic reference standard exists against which the multidisciplinary diagnosis can be validated.

此句所在的段落是谈多学科讨论（临床、影像、病理）的优缺点的，这一句是本段最后一句，讲的是多学科讨论的缺点。前面都好理解，问题在于标黑部分。此处“against”和后面的从句何解，整句连在一起又如何理解。特别是，"against”前面为什么不加一个逗号。

谢谢您的认真回答！