When we feel unwell and don't know why, a blood test is often the first step in figuring out what's wrong. But, despite what some may think, these tests don't actually identify every possible major health problem, including the one many will be most worried about - cancer.

London GP Dr Leen Tannous, who regularly posts health tips on her TikTok account, recently shared a video busting myths about why doctors carry out blood tests. She said: "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 continued standard blood tests usually check things such as white blood cells, kidney and liver function, cholesterol levels, and blood sugar, as well the thyroid and inflammatory markers such as CRP, or C-reactive protein, a chemical produced by the body in response to inflammation.

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She continued: "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 users had a range of reactions in the comments. One said: "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 wrote: "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?"

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

Another user wrote: "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 own experiences. One said: "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 wrote: "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 usually carried out to check check your general health, find out if symptoms you're having are caused by certain conditions, find out if you're more likely to get a condition, or find out how well a condition is being treated or managed. There are a number of different types of blood test depending on the condition in question.