GP explains why routine blood tests don't automatically detect cancer

When we're feeling under the weather and can't pinpoint the cause, a blood test is often our first port of call. However, contrary to popular belief, these tests don't necessarily detect every major health issue, including cancer.

London-based GP Dr Leen Tannous, who frequently shares health advice on her TikTok account, recently posted a video debunking misconceptions about why doctors order blood tests. She stated: "One of the biggest misconceptions I see as a GP is the belief that if something serious - especially cancer - was going on, it would automatically show up on a routine blood test.⁣ But that's not how blood tests work, and here's why - routine blood tests only tell us what we ask them to look for.⁣"

Dr Tannous went on to explain that standard blood tests typically examine things like white blood cells, kidney and liver function, cholesterol levels, and blood sugar, as well as thyroid and inflammatory markers such as CRP, or C-reactive protein, a chemical produced by the body in response to inflammation.

She added: "These are incredibly useful for giving us an overview of your general health but they don't screen for everything, and they don't automatically detect cancer.⁣ Some cancers don't show any changes on these routine markers at all.

"Others may show subtle hints, like low haemoglobin (sign of slow blood loss)⁣, [and] raised inflammatory markers⁣. But these findings are non-specific meaning they can be caused by many different, far more common conditions.⁣

"We can test for tumour markers, but they are not part of routine blood tests⁣, most are not screening tools⁣, [and] they are only added when a healthcare professional has a specific reason, based on your symptoms, examination, or medical history⁣.

"That's why normal routine blood tests don't always mean nothing is happening, and why symptoms always guide the investigations not the other way around.⁣ Blood tests are a tool, not a crystal ball."

TikTok viewers expressed varied responses in the comments section, reports The Mirror.

One person remarked: "The GPs need to understand this misconception! Every time I go with a problem they do bloods says every thing is fine and send me away. If I argue it they look at me like I'm crazy."

Another commented: "What's the logic behind a 'routine' blood test? Why not always test for other matters which might not be suspected at that stage, for completeness and avoidance-of-doubt?"

Meanwhile, someone else observed: "Well then maybe tumour markers should be part of routine blood tests, it's not rocket science."

A further user added: "l always think a blood test is just a way of getting rid of a patient. It comes back with 'nothing' on it and the feedback, 'nothing wrong with you' and you are left wondering what is wrong?"

Others shared their personal experiences. One revealed: "I had kidney cancer and all my blood test came back normal. Only diagnosed by an ultrasound and I was sent for that because the doctor thought it was a problem with my gallbladder. Thank god they checked while doing the gallbladder."

Another person shared: "My partner had stage 4 bowel cancer and up to the weeks leading up to his passing all his bloods apart from iron and his white blood count during active chemo were fine. Never even elevated CEA (Carcinoembryonic Antigen) levels. Drs always said based on his bloods they wouldn't have suspected he was as poorly as he was."

According to the NHS, blood tests are typically conducted to assess your overall health, determine if symptoms you're experiencing are due to specific conditions, ascertain if you're at risk of developing a condition, or monitor how effectively a condition is being treated or managed. There are various types of blood tests depending on the condition under consideration.