Colorectal carcinoma (CRC) stands as a substantial global public health concern, with an estimated 1.93 million new cases and 0.93 million deaths in 2020 (1). CRC is known to develop from precursor lesions, in most cases adenomas, through the adenoma-carcinoma sequence (2) which can be diagnosed earlier through screening even in its early stages. Through standardized early diagnosis and treatment, the 5-year survival rate for early-stage CRC could exceed 90% (1). Fecal immunochemical test (FIT) and colonoscopy screening for colorectal cancer are pivotal tools for early diagnosis of colorectal cancer (3). However, the detection performance of FIT falls short, with a miss detection rate of 9-29% for CRC and 60-75% for advanced CRC (4). FIT-positive patients are recommended to undergo colonoscopy, but colonoscopy is painful, expensive, and invasive, with the risk of complications such as perforation and bleeding. So not all FIT-positive individuals undergo regular colonoscopy follow-up (5, 6). Therefore, there is an urgent need for convenient, non-invasive, reliable, simple, and cost-effective diagnostic methods to enhance early diagnosis and screening of colorectal cancer.

The analysis of Volatile organic compounds (VOCs) has been applied as a novel and promising diagnostic technique for exploration of non-invasive colorectal neoplasia biomarker. VOCs constitute the by-products of biochemical processes within the human body and typically mirror metabolic states (7, 8). Pathological conditions precipitate aberrant metabolic processes, resulting in a marked increase in VOC production (9). Investigations into cancer-related VOCs have explored various matrices, including breath, blood, urine, saliva, and feces (10–13). Many studies have demonstrated that the applicability of VOC analysis could be used in cancer diagnosis (14–20).

The electronic nose (e-nose) emerges as an instrument equipped with a suite of sensors endowed with specificity and an adept pattern recognition system capable of discerning both simple and complex odors (21). As a relatively recent development, the e-nose has become widely accepted for detecting diseases, owing to its portability, expeditious, cost-effective, and user-friendly diagnostic capabilities, rendering it particularly suited for routine clinical applications. Multiple researchers (22–24) have substantiated the commendable diagnostic accuracy of available e-nose technologies across diverse indications. Notably, van Keulen et al. (25) analyzed exhaled breath from patients with CRC and advanced adenomas (AAs), proving that the Aeonose electronic nose can distinguish CRC and AAs from controls. Additionally, de Meij et al. (26) reported an e-nose sensitivity of 0.85 and a specificity of 0.87 in CRC detection.

Our article aims to systematically review published studies on VOC analysis and e-nose technology concerning colorectal cancer (CRC) detection. Furthermore, we aim to compare their diagnostic performance, with the aspiration of offering a valuable reference for the application of diagnostic techniques in CRC diagnosis.

We conducted a systematic review and meta-analysis to evaluate VOC analysis and electronic nose in detecting colorectal cancer, aiming to compare the diagnostic accuracy and clinical application value of these two methods. Pooled analysis of VOC and electronic-nose studies demonstrated high diagnostic accuracy for CRC detection, with a pooled sensitivity of 0.88 and specificity of 0.85 for VOC analysis and a sensitivity of 0.87 and specificity of 0.78 for e-nose studies. The visually assessed SROC curves indicated clinical accuracy, with VOC analysis and e-nose having SROC curves of approximately 0.93 and 0.90, respectively, both close to 1, signifying superior accuracy and diagnostic efficacy in CRC diagnosis. These findings align with prior reviews (22, 59, 60), but the notable heterogeneity between studies and the identified high risk of bias warrant cautious interpretation. The heterogeneity was largely due to the sample media and the analytical methods used.

Subgroup analyses revealed that breath samples in VOC analysis and urine and breath samples in e-nose studies exhibited higher sensitivity or specificity. Breath sampling is easily performed and well-received by patients, and urine samples, boasting high sensitivity and specificity, emerge as valuable alternatives. Recent meta-analysis evaluated the performance of the combined FIT and urinary. The findings revealed that the combined FIT-VOC approach could detect 33% more cases of colorectal cancers (60). Chandrapalan S et al. (61) showed that the combination of FIT and VOC can be a better triage tool, for CRC in patients with lower gastrointestinal symptoms than FIT alone.

Due to the lack of standardization in sample collection, handling, and storage, technical barriers exist in measuring and analyzing various VOC characteristics during sampling, whether it involves alveolar air, urine, or feces. In several studies, exhaled breath was collected into a bag and subsequently analyzed (28, 29, 33, 38–40, 42, 50). The use of bag collection aligns more closely with real-world medical applications. However, this approach may be influenced by several factors, including interference from ambient VOCs, the material used for collection, and the impact of temperature, humidity, and storage time on specimens (62). For breath samples, it is essential to examine them within 6 hours of the collection’s conclusion to ensure test accuracy (63). Therefore, developing methods for the collection, transmission, and handling of breath samples is crucial for the success of this approach. Some studies have indicated that the diagnostic accuracy of fecal and urine VOCs is not significantly affected by storage time (20 months for fecal and 12 months for urine VOCs) (64, 65).

Urine samples are ideal detection medium because they have limited confounding factors compared to breath samples which is influenced by smoking or fecal samples influenced by diet. Further research should standardize the method of collection of such samples and investigate the effects of potential confounding factors.

Among all studies, only six reported on CRC stages, indicating limited generalizability and clinical applicability. Multi-center validation studies of the diagnostic performance of VOCs on early stages of CRC and its precursor lesions (adenomas or not) is warranted, which could reduce the incidence of CRC.

It has been demonstrated that various factors, such as age, gender, smoking, alcohol consumption, coffee intake, and the consumption of stimulating foods like leeks and garlic, as well as comorbidities and medication, may influence the composition of VOCs in exhaled breath (66). However, only a few studies considered confounding or modifying effects, limiting the validity and reliability of the results. Therefore, future studies should account for the impact of such factors on breath prints during the design phase.

Gas chromatography-mass spectrometry (GC-MS), a traditional method for VOC analysis, is a highly standardized technique providing qualitative and quantitative information on exhaled VOCs (67, 68). In this study, TD-GC-MS demonstrated high sensitivity and specificity in detecting colorectal cancer, while GC-MS exhibited improved specificity but suboptimal sensitivity. The use of GC-MS and newer mass spectrometry technology devices remains the gold standard for identifying specific VOCs for analysis. However, GC-MS technology is costly and complex, with long analysis times, and it demands a high level of expertise from operators.

Based on sensors, electronic nose technology serves as a novel analytical method for disease diagnosis, offering the advantages of being cost-effective, user-friendly, portable, sensitive, and responsive. Nevertheless, there are existing shortcomings that require refinement in the application of e-nose in clinical practice. Unlike GC-MS and other techniques, e-nose lacks the precision to measure specific types and composition ratios of components in VOCs (24). It also cannot identify specific pathophysiological pathways or therapeutic targets. Furthermore, as the e-nose relies on arrays of gas sensors to distinguish and identify response spectra of mixtures composed of multiple VOCs, the diverse sensor types with distinct signal responses prevent the integration of results from one e-nose with different devices or sensor types (69). Van der Sar IG (70) recommends the establishment of a comprehensive worldwide shared database encompassing patient characteristics and other pretest probabilities.

Various algorithms and methods were employed to analyze VOCs in this study, with PLA-DA and logistic regression analysis emerging as the most commonly used approaches. However, the majority of studies fail to elucidate the rationale behind selecting a specific machine learning model for analysis, only reporting the highest accuracy value, thereby impacting the reliability of the results. Additionally, studies with small sample sizes may compromise the reported accuracy. Few studies have conducted external validation to affirm the validity and reliability of these findings. Consequently, large, multi-center external validation studies should be conducted in the future to explore the applicability and reproducibility of the results in different study settings and among diverse target populations.

Based on our meta-analysis, VOC analysis and e-nose technology show promise in the detection of CRC. However, several milestones must be achieved in colorectal cancer detection with these two non-invasive methods before clinical implementation. Firstly, for patients presenting with common non-specific symptoms, which may be an early indication of CRC, an exhaled breath test or a urine test or FIT+VOC could serve as screening tool. Secondly, electronic nose could be utilized in primary care units and community healthcare centers for mass screening of various intestinal diseases due to their portability, ease of use, cost-effectiveness, speed, and independence from specialized technicians. Thirdly, the identification of colorectal cancer-specific VOC biomarkers and combinations of biomarkers for colorectal cancer diagnosis is still necessary. This requires comprehensive metabolomics studies to elucidate the production of endogenous VOCs and the metabolic transformation of exogenous VOCs in colorectal cancer, aiding in the identification of VOC markers for cancer. Finally, large, multi-center external validation trials should be conducted to verify the generalizability and reproducibility of the results in different research settings and at different stages of CRC.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

QW: Formal analysis, Funding acquisition, Supervision, Writing – original draft. YF: Investigation, Project administration, Writing – original draft. ST: Methodology, Writing – review & editing. ZL: Data curation, Project administration, Writing – review & editing. RZ: Data curation, Investigation, Writing – review & editing. YR: Methodology, Project administration, Writing – original draft. YJ: Project administration, Software, Writing – original draft. XH: Supervision, Validation, Writing – review & editing.