Introduction:

For decades, blood has been the gold standard for molecular diagnostics. But what if the same or even more information could be obtained from something far simpler and entirely non-invasive: saliva?

Today, saliva has emerged as a powerful diagnostic fluid capable of revealing the molecular signatures of not only oral diseases but also systemic, infectious, autoimmune, and even malignant disorders. Advances in molecular pathology and high-throughput omics technologies have transformed saliva into a “liquid biopsy,” mirroring the physiological and pathological state of the entire body.

Why Saliva?

Saliva is a complex mixture of water, electrolytes, enzymes, mucins, hormones, DNA, RNA, microRNAs (miRNAs), proteins, metabolites, and extracellular vesicles. Unlike blood, its collection is:

• Non-invasive – No needles or discomfort.

• Safe and repeatable – Ideal for large-scale screening and monitoring.

• Cost-effective – Requires minimal equipment and trained personnel.

• Stable – Certain biomarkers remain intact under proper storage and processing conditions.

Because it reflects both local oral and systemic molecular changes, saliva has become a window into multiple disease processes.

Head & Neck Diseases:

Salivary diagnostics have transformed the landscape of oral pathology. In oral squamous cell carcinoma (OSCC), several molecular signatures including TP53 mutations, EGFR overexpression, and upregulation of miR-21, miR-31, and IL-8 can be detected in saliva, enabling early detection and recurrence monitoring.

In oral potentially malignant disorders like leukoplakia or lichen planus, salivary miR-155, cytokines (IL-6, TNF-α), and oxidative stress markers (8-OHdG) provide insight into disease activity. Similarly, periodontal disease is diagnosed through bacterial DNA (Porphyromonas gingivalis, Tannerella forsythia) and host response biomarkers like IL-1β and MMP-8.

Even Sjögren’s syndrome, once reliant on invasive salivary gland biopsies, now finds diagnostic potential in salivary RNA profiles particularly anti-Ro/SSA and anti-La/SSB–associated transcripts and miR-574-5p expression.

Infectious Diseases: Detecting Pathogens Without Blood:

During the COVID-19 pandemic, saliva-based RT-PCR became a global standard for SARS-CoV-2 detection. This success accelerated research into other pathogens detectable via saliva:

• HIV-1 RNA and antibodies.

• Hepatitis B & C viral nucleic acids.

• Human papillomavirus (HPV-16/18) DNA in oropharyngeal cancers.

• Mycobacterium tuberculosis IS6110 PCR.

• Epstein–Barr virus (EBV) and Cytomegalovirus (CMV) DNA in immunocompromised. patients.

These assays highlight saliva’s potential as a diagnostic matrix for infectious disease surveillance and personalized patient monitoring.

Systemic and Neurological Disorders:

Beyond infection, salivary biomarkers are proving useful in detecting systemic conditions:

Such biomarkers could enable early, non-invasive monitoring of disease progression and treatment efficacy especially valuable for chronic disorders requiring frequent testing.

Cancer Diagnostics Beyond the Oral Cavity:

Saliva harbors genetic and epigenetic signals shed from distal tumors, making it useful for non-oral cancer detection. Examples include:

• Breast cancer: miR-21, miR-1246, and CEA mRNA.

• Lung cancer: EGFR mutations, miR-31, miR-200a.

• Pancreatic cancer: KRAS mutations, MUC1 mRNA.

• Colorectal cancer: p16 and MLH1 methylation.

• Prostate cancer: PCA3 and TMPRSS2-ERG transcripts.

Emerging salivary exosome sequencing and lab-on-chip PCR systems promise real-time cancer screening in primary care settings.

Autoimmune, Endocrine, and Genetic Disorders:

In systemic lupus erythematosus (SLE), miR-21 and miR-146a reflect immune dysregulation, while rheumatoid arthritis can be tracked via IL-6 mRNA and miR-155 expression. Cystic fibrosis gene mutations (CFTR ΔF508) and thyroid disorder markers (TSH, TPO, miR-146b) are also measurable in saliva, reflecting the tissue’s diagnostic versatility.

The Future of "Saliva-Omics":

The next decade will see integration of multi-omics approaches genomics, transcriptomics, proteomics, and metabolomics from a single salivary sample. Microfluidic “lab-on-a-chip” platforms and AI-driven biomarker analysis will soon make it possible to diagnose complex diseases in minutes, at the chairside or bedside.

In precision medicine, saliva will play a central role in risk prediction, early detection, and treatment response monitoring, especially in oncology, autoimmune, and neurodegenerative disorders.