Researchers said measuring DNA sections in cancer cells gave an "accurate indicator" of disease progression.
Shorter DNA structures can leave chromosome ends exposed, accelerating cancer progression and drug resistance.
The team believe the study could help doctors in choosing the most effective treatment for individual patients.
Researchers from the university's School of Medicine said the tests can be used to predict the outcomes of patients with two different types of blood cancer - the bone marrow cancer myeloma and pre-leukaemia myelodysplastic syndromes (MDS).
The latter is a bone marrow disorder often leading to life-threatening bone marrow failure and even acute myeloid leukaemia (AML).
Results from two studies, which were funded by the charities Bloodwise and Cancer Research UK, have been published in the British Journal of Haematology.
The researchers analysed samples from 134 myeloma patients, 80 MDS patients and 95 AML patients as part of the study.
Prof Duncan Baird, who led the research, said: "Our research provides strong evidence that shortening of telomeres [the DNA sections] plays a vital role in the progression of these blood cancers and that a significant number of patients should be receiving different levels of treatment.
"The next step is to assess telomere length in larger studies to establish how it can be integrated into existing assessments that predict patient outcome."
A new test that could hold the key to predicting blood cancer patients' survival has been developed by Cardiff University.