Something as simple as a urine test may be able to distinguish between aggressive and low-grade bladder cancers, allowing doctors to tailor personalised treatments, say scientists.
Researchers measured levels of a protein shed by bladder tumours in 600 patients.
They found that higher amounts of the protein, EpCAM, in the urine were associated with more aggressive cancers.
Study author Dr Douglas Ward, from the University of Birmingham, said: "This protein could be used to help doctors to decide what the best course of investigation or treatment for the patients is, and may prevent unnecessary delays.
"We've known for some time that the protein EpCAM is released from some tumour cells but it wasn't clear whether it would be useful as a way to decide the best investigation and treatment for patients suspected of having bladder cancer.
"We are now planning further studies to test the benefits of urine biomarker testing to patients and the NHS."
Each year around 10,300 bladder cancers are diagnosed in the UK and 5,000 people die from the disease.
In many cases, the tumours are superficial and do not pose a significant risk to life.
Invasive cancers that spread into the surrounding muscles of the bladder are less common but can be fatal.
Martin Ledwick, head information nurse at Cancer Research UK, which funded the study, said: "This research has shed new light on a protein that we've known for some time is linked to certain types of cancer.
"Developing a urine test to work out how aggressive or advanced a patient's tumour is could replace the need for more invasive and costlier tests used by doctors at the moment."
The research is published in the British Journal of Cancer.