Saliva samples in the spin column-based saliva DNA purification/extraction kit contain large amounts of carbohydrates (from the highly glycosylated protein mucin). Although the protein is removed during extraction, a small amount of this carbohydrate remains. Carbohydrates absorb very strongly at 230 nm, so even small amounts of carbohydrates can increase the 230 reading, resulting in a poor ratio.
These carbohydrates do not affect subsequent use, so A260/A230 is not a valuable method for assessing suitability for subsequent use of DNA extracted from saliva samples. The calculation of the A260/A280 can accurately measure the purity of DNA isolated from saliva,
The absorption ratio at 260 Nm vs 280 nm is because proteins (incredibly aromatic amino acids) absorb light at 280 nm. When extracted, the average A260/A280 ratio ranges from 1:6 to 1:9. These ratios usually indicate that the DNA sample will perform well in your downstream application as all your other quality control metrics pass (high molecular weight on the gel, acceptable concentrations by fluorescence-based quantification).
Medical Professionals Get High-Quality and High-Molecular Weight DNA from Saliva
The RNA samples will co-purify with the DNA when extracting DNA from the saliva samples. The RNA will not affect the downstream applications. It includes PCR, SNP Genotyping, WES or WGS. The experts can remove them, but it does affect the quantification of the DNA.
When determined by absorbance, the total amount of nucleic acids in the sample (RNA and DNA) is measured. Therefore, the amount of DNA in the sample may be overestimated, resulting in a lower yield due to too little DNA in your assay. Fluorescence-based quantification, such as Picogreen or Qubit, accurately measures DNA in a saliva sample.
Saliva Can Easily Take the Place of Blood in DNA Analysis
The golden standard for DNA quality assessment is blood collection. Blood sample collection is an established practice across all health clinics, hospitals, labs and test centres. However, many people might not know they can submit their saliva sample instead of a blood sample in the saliva nucleic acid solution /purification/extraction kit for genomic DNA analysis. Using saliva to test the genomic DNA analysis is a proven method.
Saliva collection kits have a design that can stabilise the high molecular weight DNA by constraining degradation and curbing bacterial growth. Most of the DNA obtained with Oragene is > 23 kb in size, and the number of bacteria has no practical importance because the majority is of human origin (only 11.8 bacteria on average). Multiple studies confirm that saliva-extracted DNA produces the most intact DNA that performs as well as blood in the most demanding applications, including microarrays and sequencing (targeted and whole genome).
