Over the past decade, various liquid biopsy techniques have emerged as viable alternatives to the analysis of traditional tissue biopsy samples. Such surrogate ‘biopsies’ offer numerous advantages, including the relative ease of obtaining serial samples and overcoming the issues of interpreting one or more small tissue samples that might not reflect the entire tumour burden. To date, the majority of research in the area of liquid biopsies has focused on blood-based biomarkers, predominantly using plasma-derived circulating tumour DNA (ctDNA). However, ctDNA can also be obtained from various non-blood sources and these might offer unique advantages over plasma ctDNA. In this Review, we discuss advances in the analysis of ctDNA from non-blood sources, focusing on urine, cerebrospinal fluid, and pleural or peritoneal fluid, but also consider other sources of ctDNA. We discuss how these alternative sources can have a distinct yet complementary role to that of blood ctDNA analysis and consider various technical aspects of non-blood ctDNA assay development. We also reflect on the settings in which non-blood ctDNA can offer distinct advantages over plasma ctDNA and explore some of the challenges associated with translating these alternative assays from academia into clinical use.

在过去的十年中，各种液体活检技术已经成为传统组织活检样本分析的可行替代方案。这种替代“活检”具有许多优点，包括相对容易获得连续样本，并克服解释可能无法反映整个肿瘤负荷的一个或多个小组织样本的问题。迄今为止，液体活检领域的大部分研究都集中在血液生物标志物上，主要使用血浆来源的循环肿瘤 DNA (ctDNA)。然而，ctDNA 也可以从各种非血液来源获得，这些可能比血浆 ctDNA 具有独特的优势。在这篇综述中，我们讨论了非血液来源 ctDNA 分析的进展，重点关注尿液、脑脊液、胸膜或腹膜液，但也考虑了 ctDNA 的其他来源。我们讨论这些替代来源如何对血液 ctDNA 分析发挥独特但互补的作用，并考虑非血液 ctDNA 检测开发的各个技术方面。我们还反思了非血液 ctDNA 比血浆 ctDNA 具有明显优势的情况，并探讨了将这些替代检测方法从学术界转化为临床应用所面临的一些挑战。