Many Britons assume that routine blood tests can identify all serious health conditions, but a London GP has revealed this common misconception could be putting people at risk.
The limitations of standard blood testing
Dr Leen Tannous, a GP based in North West London, has used her TikTok platform to educate the public about what blood tests can and cannot reveal about their health. The doctor emphasised that cancer frequently doesn't show up on routine blood tests, contrary to widespread belief.
"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," Dr Tannous explained in her viral video. "But that's not how blood tests work."
What routine blood tests actually check
Standard blood tests typically examine specific markers including white blood cell counts, kidney and liver function, cholesterol levels, blood sugar, thyroid function and inflammatory markers like CRP (C-reactive protein). While these provide valuable information about general health, they don't serve as comprehensive screening tools for all conditions.
"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," Dr Tannous clarified. "Some cancers don't show any changes on these routine markers at all."
When blood tests might hint at cancer
The GP acknowledged that sometimes blood tests can show subtle indicators that might suggest further investigation is needed. These can include low haemoglobin levels indicating slow blood loss, or raised inflammatory markers. However, these findings are non-specific and could be caused by numerous common conditions unrelated to cancer.
Dr Tannous also addressed the topic of tumour markers, explaining that these are not included in routine blood tests. "Most are not screening tools," she noted. "They are only added when a healthcare professional has a specific reason, based on your symptoms, examination, or medical history."
The doctor concluded with an important reminder: "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."
Public reaction and personal experiences
The video sparked significant discussion among TikTok users, with many sharing their own experiences and concerns about the healthcare system.
One commenter expressed frustration with how test results are handled: "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."
Others questioned why more comprehensive testing isn't standard practice. "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?" one user posted.
Several commenters shared personal stories that highlighted the limitations of blood testing. "I had kidney cancer and all my blood test came back normal," one revealed. "Only diagnosed by an ultrasound and I was sent for that because the doctor thought it was a problem with my gallbladder."
Another shared a particularly sobering experience: "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 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 performed to assess general wellbeing, identify the cause of specific symptoms, determine risk factors for certain conditions, or monitor how effectively existing conditions are being managed. The healthcare service emphasises that different types of blood tests exist depending on what conditions are being investigated.
