Prostate cancer: A new at-home saliva test beats blood test

A new at-home spit test is better at predicting men's prostate cancer risk than current hospital tests, a new study shows. The test assesses 130 genetic variants in people's DNA to provide a risk score of their developing prostate cancer.

The study found the saliva-based test was more accurate at screening for prostate cancer than existing, blood tests, which measure the levels of a protein called prostate-specific antigen (PSA).

Scientists say the new genetic test — known as a polygenic risk score (PRS) — could help turn the tide on prostate cancer, which globally claims the lives of 400,000 men every year.

"This study is the strongest evidence to date on the clinical utility of a polygenic score for prostate cancer screening," said Michael Inouye, an expert in population health at University of Cambridge, UK.

"It shows that a polygenic score can improve early detection of clinically significant prostate cancer, including those warranting radical treatment."

Prostate cancer risk scores: PSA vs. PRS

The study, published in the New England Journal of Medicine, calculated what's called a polygenic risk score of more than 6,000 men in the UK.

A polygenic risk score (PRS) is a calculation of the likelihood someone will develop a disease based on unique variations in their DNA, along with other metrics, such as age and gender.

Using the saliva test, researchers said they had been able to identify 745 men with high polygenic risk scores. Those men were invited for further screening tests, including Magnetic Resonance Imaging (MRI) and prostate biopsy. 

The tests helped to identify prostate cancer in 187 participants. The cancer would not have been detected in 74 of the men using current diagnostic methods that measure PSA levels, and MRI results, said the researchers.

Other experts, who were not involved in the research, said the new saliva test could be used as an additional screening tool. Testing DNA for prostate cancer had the benefit of reducing the number of false positive results and detecting a higher proportion of aggressive cancers than current standards of care, they said.

Benefits of genetic prostate cancer test limited to European populations

While the results were promising, Dusko Ilic, a biomedical expert at King's College London, UK, said there are several caveats which would limit the study's impact.

First, the cost-effectiveness of the screening had not yet been fully evaluated, Ilic said.  

It was also unclear whether the results were relevant for men around the world. The work was made possible with extensive research into genetic risks for prostate cancer. However, it was only validated in men from European populations, which limits the applicability of the study to other populations, experts said.

Research on genetic risk for prostate cancer for other populations is currently limited, which is why "a similar risk score for men of Black African or Caribbean ancestry is urgently needed. We know that these men have a much higher prostate cancer risk than those of European ancestry," said Britta Stordal, an expert in cancer research at Middlesex University, UK.

How long until the saliva test becomes available?

The researchers said the limitations of the study needed to be addressed before prostate cancer screening programs using the saliva test could be rolled out. 

"While PRS could supplement existing screening in high-risk individuals, the evidence is insufficient to recommend a standalone screening program based solely on PRS at this time," said Ilic.

This means more research, more testing, and more investment. How long will this take? Referring to the UK — where the research was conducted — Inouye said it would "likely be years" before polygenic scores were routinely used to screen for prostate cancer.