The “liquid” in liquid biopsy most commonly refers to blood. However, the human body contains many other fluids and biological matrices that can be collected easily and non-invasively, whilst providing an important source of molecular information.
Saliva is one of the easiest to collect biofluids and is already used routinely for diagnostic purposes. It is a promising source of cancer biomarkers and has already been demonstrated to contain multiple different tumour biomarkers, particularly for head and neck cancers.
The standout feature of saliva-based liquid biopsy is the ease of sampling, which can be performed without specialist training or expensive equipment. This makes saliva a good candidate in the application of liquid biopsy in disease monitoring, as multiple repeat samples can easily be taken. The qualities of saliva also open up the possibility of point-of-care testing, with individuals collecting their own samples to be sent for analysis.
Interest in saliva-based cancer molecular diagnostics has also grown rapidly, with potential shown in disease detection and monitoring. Saliva is a particularly rich source of biomarkers relating to head and neck cancer.
Saliva-based liquid biopsy has also expanded to less obvious tumour types. Novel liquid biopsy platforms targeting saliva have been developed, such as electric field–induced release and measurement (EFIRM) for the detection of EGFR mutations directly from non-small cell lung cancer (NSCLC) patient saliva.
Sputum is the combination of saliva, phlegm, and mucus derived from the upper respiratory tract. Although sputum has many similarities with saliva, it has not seen as much development within liquid biopsy. Sampling of sputum can be performed using spontaneous production or through induction by inhalation of warm saline aerosol.
Urine is also a liquid that is straightforward to collect, with large volumes available compared to other types of liquid biopsy. As with saliva, the ease of collecting urine makes it suitable for disease monitoring using repeat samples. Compared to other fluids, urine contains smaller numbers of circulating cells, but it is a comprehensive source of other biomarkers such as circulating nucleic acids, proteins and extracellular vesicles.
As expected, the majority of urine liquid biopsy research has focused on urological cancers. Many of the biomarkers released from urological cancers are likely to drain directly into the urinary tract, making urine an ideal matrix for their detection and analysis.
Multiple studies have also explored urine liquid biopsies for less obvious cancer types. Mutations in the EGFR gene have been analysed in ctDNA derived from NSCLC patient urine. Several miRNAs have also been highlighted as potentially useful urine biomarkers for the early detection of gastric, colorectal, endometrial and ovarian cancer.
Faeces consists of water, bacteria, food components and metabolites. Changes in the intestine are reflected in the composition of faeces, with hundreds of different types of metabolites (amino acids, lipids, triglycerides) discovered in stool samples.
The presence of blood in a faecal occult blood test is used as a pre-cursor to a more invasive procedure such as a colonoscopy. By integrating a molecular component, the analysis of faeces could be translated into a sensitive and specific test that may reduce the need for these invasive procedures.
Sweat is secreted from the sweat glands onto the surface of the skin and its primary role is the thermoregulation of body temperature. In doing so, sweat also removes waste products from the body and is therefore a viable source of molecular biomarkers. Candidate biomarkers present in sweat include electrolytes, metabolites, hormones, proteins, nucleic acids, carbohydrates and urea.
The collection of sweat comes with the caveat of inducing sweat production in the individual it needs to be collected from. This can be done either by physical activity, usually at an increased room temperature, or through chemical means by applying alcohol and/or pH7 phosphate buffer to the skin.
Cerebrospinal fluid (CSF)
The gold standard for diagnosing and monitoring central nervous system (CNS) tumours is via imaging techniques. However, imaging doesn’t provide molecular information that can be crucial to therapeutic decision making. To compound matters, surgical sampling of malignancies of the central nervous system can be challenging. The tumours can be in hard-to-reach areas, and this may limit our ability to correctly characterise specimens.
It has recently been demonstrated that the detection of circulating tumour biomarkers in CSF can be used to characterise and monitor CNS malignancies. ctDNA is highly present in CSF (in larger amounts than plasma). Its analysis can inform therapeutic decision making and help track tumour evolution.
Bone marrow has been explored as a rich source of biomarkers from multiple cancer types including breast, prostate, lung and colorectal. Pleural effusions, collected from advanced lung malignancies, have shown to have increased levels of CTCs and extracellular vesicles.
The continuous growth and development of liquid biopsy has led to researchers exploring lesser well-known liquids in the pursuit of non-invasive diagnosis. One such instance is the use of the aqueous humor in the eye to address diagnostic problems within childhood retinoblastoma.