28S/18S Ratio Detection: A Complete Guide to RNA Integrity Assessment

The 28S/18S ratio is one of the most widely used indicators of RNA integrity, providing a rapid and reliable measure of total RNA quality before downstream molecular experiments. Professional 28S/18S ratio detection services help researchers confirm that RNA samples remain intact and suitable for applications such as RNA sequencing, microarray analysis, and qRT-PCR. This article explains the principle, sample requirements, testing methods, and applications of 28S/18S ratio detection.

Technical Overview

The 28S/18S ratio detection is a fundamental quality control procedure used to evaluate the integrity of total RNA extracted from biological samples. In eukaryotic cells, ribosomal RNA (rRNA) constitutes approximately 80–85% of total cellular RNA, with the two major species being 28S rRNA (approximately 4.7–5.0 kb in mammals) and 18S rRNA (approximately 1.9 kb). When total RNA is separated by electrophoresis and visualized with a fluorescent dye, these two rRNA species appear as distinct bands.

For high-quality, intact eukaryotic RNA, the 28S rRNA band should appear roughly twice as intense as the 18S rRNA band, yielding a 28S/18S ratio of approximately 2.0 or higher. This reflects the natural stoichiometry of ribosomal subunits. As RNA degrades, the larger 28S rRNA is preferentially cleaved by RNases, causing the ratio to decline and smear patterns to appear between and below the two bands. Therefore, the 28S/18S ratio serves as a direct visual and quantitative indicator of RNA degradation.

In addition to the ratio itself, a comprehensive RNA quality assessment typically includes RNA concentration, purity ratios (A260/A280 and A260/A230), and, when automated capillary electrophoresis platforms are used, an RNA Integrity Number (RIN) value. Together, these parameters determine whether a sample is suitable for demanding downstream applications such as whole-transcriptome sequencing.

Testing Samples

Animal Tissues

Fresh or frozen animal tissues, including liver, brain, heart, kidney, spleen, muscle, and tumor specimens, are commonly submitted for 28S/18S ratio detection. Tissues should be snap-frozen in liquid nitrogen immediately after collection and stored at −80°C to prevent RNase-mediated degradation. RNase-rich tissues such as pancreas and spleen require particularly rapid processing.

Plant Tissues

Plant samples, including leaves, roots, stems, seeds, and fruits, can be analyzed for 28S/18S ratio. Plant tissues often contain polysaccharides, polyphenols, and secondary metabolites that interfere with extraction and electrophoresis, so specialized extraction protocols may be required. Note that in plants, the larger rRNA species is often designated 25S rather than 28S, but the analytical principle is identical.

Cultured Cells

Mammalian cell lines, primary cells, stem cells, and microbial cultures are suitable sample types. Cells should be pelleted, washed, and either processed immediately or stored frozen to preserve RNA integrity.

Blood and Clinical Specimens

Peripheral blood, leukocyte fractions, buffy coats, fresh-frozen tissue biopsies, and FFPE tissue sections can be evaluated. Clinical specimens typically require standardized handling and documentation, and FFPE samples usually show substantial degradation with low 28S/18S ratios due to formalin crosslinking and fragmentation.

Extracted RNA Samples

Researchers who have already isolated total RNA may submit purified RNA directly for 28S/18S ratio analysis. Samples should be dissolved in RNase-free water or low-salt buffer, kept on ice during shipping, and transported with dry ice for long-distance delivery.

Testing Items

A standard 28S/18S ratio detection service typically includes the following analytical items:

• 28S/18S rRNA band ratio determination
• RNA integrity assessment by electrophoretic profile (band sharpness, smear analysis)
• Total RNA concentration measurement
• Purity assessment via A260/A280 ratio (target: approximately 1.8–2.1)
• Purity assessment via A260/A230 ratio (target: approximately 2.0–2.2)
• RIN (RNA Integrity Number) value, when using microfluidic capillary electrophoresis platforms
• DV200 value (percentage of RNA fragments longer than 200 nucleotides) for degraded or FFPE samples
• Comprehensive quality report with electropherogram or gel image and pass/fail recommendation

Testing Methods

Agarose Gel Electrophoresis

Traditional agarose gel electrophoresis remains a straightforward and cost-effective method for 28S/18S ratio detection. Total RNA is loaded onto a 1.0–1.5% agarose gel, separated under denaturing or non-denaturing conditions, and visualized using intercalating dyes such as ethidium bromide, SYBR Safe, or GelRed. Band intensities are quantified by densitometry using image analysis software to calculate the 28S/18S ratio. A ratio of 1.8–2.0 or higher, with sharp bands and minimal smearing, indicates intact RNA.

Microfluidic Capillary Electrophoresis

Automated platforms such as the Agilent Bioanalyzer 2100, Agilent TapeStation, and AATI Fragment Analyzer separate RNA fragments through microfluidic chips and detect them by laser-induced fluorescence. These systems generate digital electropherograms, calculate the 28S/18S ratio automatically, and assign a RIN value from 1 (most degraded) to 10 (completely intact). RIN values of 7 or above are generally recommended for RNA-seq library preparation, while RIN 8 or above is preferred for long-read and small RNA applications.

UV-Vis Spectrophotometry

Instruments such as the NanoDrop are used alongside electrophoresis to measure RNA concentration and assess purity through absorbance ratios. A260/A280 values near 2.0 indicate minimal protein contamination, while low A260/A230 values suggest contamination with phenol, guanidine salts, or carbohydrates that may compromise downstream enzymatic reactions.

Fluorometric Quantification

RNA-specific fluorescent dyes (for example, Qubit RNA HS assay) provide accurate concentration measurements even in the presence of DNA, free nucleotides, or solvent contaminants, complementing the integrity data obtained from electrophoretic analysis.

Applications

28S/18S ratio detection is an essential quality control step in a wide range of research and clinical contexts:

• RNA sequencing (RNA-seq) library preparation, where input RNA quality directly determines data quality
• Microarray gene expression profiling, which requires high-integrity RNA for reliable hybridization
• Quantitative real-time PCR (qRT-PCR) and RT-PCR assay validation
• Northern blot analysis of transcript size and abundance
• Full-length cDNA library construction and clone screening
• Long-read sequencing and direct RNA sequencing projects
• Single-cell transcriptomics, where low-input samples demand verified RNA quality
• Biobank and clinical tissue repository quality assurance programs
• Pharmaceutical manufacturing and drug development QC for RNA-based products, including mRNA vaccines
• Gene knockout and transgenic organism characterization studies
• Environmental and agricultural stress-response research involving transcriptome analysis

FAQ

What is considered an acceptable 28S/18S ratio?

For mammalian total RNA, a ratio of approximately 2.0 is the classical benchmark for fully intact RNA. In practice, ratios between 1.5 and 2.0 are often acceptable for routine applications such as qRT-PCR, whereas ratios of 2.0 or higher with corresponding high RIN values are preferred for RNA-seq and other demanding applications. Plant RNA typically yields a slightly lower ratio due to differences in rRNA structure and content.

Why does my 28S/18S ratio keep coming out low?

Common causes include endogenous or exogenous RNase contamination, delayed sample freezing, repeated freeze-thaw cycles, harsh extraction conditions, residual phenol or ethanol in the final RNA preparation, and improper storage temperature. Improving sample handling speed, using RNase-free reagents and consumables, and storing RNA at −80°C in aliquots typically resolves the issue.

How does the 28S/18S ratio relate to RIN?

The RIN is an algorithmic score derived from the entire electrophoretic profile generated by capillary electrophoresis systems, whereas the 28S/18S ratio considers only the two major rRNA peaks. RIN provides a more comprehensive and reproducible assessment of integrity and has largely supplemented the ratio as the industry standard; however, the 28S/18S ratio remains valuable, especially when gel electrophoresis is the available method.

Can degraded RNA still be used for experiments?

It depends on the application. Partially degraded RNA with low 28S/18S ratios can still yield valid qRT-PCR results when short amplicons are targeted. For FFPE samples, DV200 rather than RIN is used, with values above 50% generally considered usable for RNA-seq. Highly degraded RNA is unsuitable for full-length transcript analysis and long-read sequencing.

How should samples be prepared and shipped for testing?

Tissues should be flash-frozen in liquid nitrogen and shipped on dry ice. Extracted RNA should be provided at a minimum concentration (typically 20–50 ng/µL or higher) in RNase-free water or TE buffer, clearly labeled, and accompanied by sample information including species, tissue type, extraction method, and storage history. Avoid repeated freeze-thaw cycles and minimize transit time.

How long does the detection process take?

Standard 28S/18S ratio detection using agarose gel electrophoresis or chip-based capillary electrophoresis is typically completed within 1–3 working days after sample receipt, with digital electropherograms, gel images, and a full quality report delivered to the client.