'Exciting' new cancer blood test results hide a worrying problem:

Progress in reducing the global toll of cancer is still slow, but a new blood test has raised an extraordinary level of optimism.

Progress in reducing global cancer deaths has been painfully slow, but a new blood test has raised an unusual level of hope.Researchers say it could one day make routine screening more effective by catching cancer earlier, when treatment has the best chance of saving lives.

The Galleri blood test, developed by US company Grail, is the latest product to attract worldwide attention. After initial results were described as "exciting" by researchers

A press release said the test, which is currently being tested by the NHS, can pick up signals from 50 cancers and correctly identify the disease in 62% of people who test positive.

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It also appears to be very accurate at ruling out cancer, with a 99.6% success rate among people who do not have the disease.At first glance, these headline numbers seem like significant progress.

But before we get to the champagne, it's worth taking a closer look at what these numbers actually mean.Initial promise doesn't always translate into real-world performance.

The Pathfinder 2 study involved 23,161 people over age 50 from the United States and Canada with no previous cancer diagnosis.resulting in information that is widely disseminated todayOf the 216 participants who tested positive,It was later found that 133 cases had cancer, giving a "positive predictive value" (PPV) of 62 percent that was widely reported.

This measure answers an important question: "If I test positive, how likely is it that I actually have cancer?" This also means that 38% of positive results were false alarms.

The feature - how many times the test prevents the diagnosis of cancer - is considered important, because the anxiety and anxiety of treatment are obtained with false results.

Here the test performed well, with 99.6% of people without cancer having a proper negative result.

However, even this large number matters.If more than 50 people tested in the UK, that's more than 26 million people, the same number would still result in more than 100,000 false positives.

The strong issues are the sensitivity of the actual cancer tests that actually detect them.The results of these measurements were 40.4 percent, meaning the test missed three out of every five cancers that appeared during the following year.

Test results:

The most widely reported image

This shortcoming may disappoint those expecting a comprehensive screening tool.It also increases the risk of patients being falsely reassured by a negative result, which could delay diagnosis.

Statistical reports of PPV, Specificity, and Sensitivity are not continuous fractions, each calculated with uncertainty.Also, they are small controlled trials in small controlled trials and the real world may be less.

So where does this leave Galleri's test?It may become a useful addition to future screening programs if patients or physicians do not consider definitive negative results.

But low sensitivity means many cancers will still be missed in their current form. The test is also expensive - $949 (£723) in the US - and there is currently no evidence that its widespread use will reduce cancer deaths.

Early information is encouraging, but perhaps there is no need to get excited.Technology may be a step forward, but it's not a watershed solution.

John Ferguson, Senior Lecturer in Statistics, University of Galway

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