There is no such thing as a 'blood test for cancer'
Published 15/09/2015 | 00:00
When Bella, a thirteen year old Labrador Retriever, became picky about eating her dinner, and stopped enjoying walks as much as usual, her owner knew that there was something amiss, so he brought her to see me.
There are three stages to an investigation of any sick pet. First, a detailed history of the pet's problem needs to be taken from the owner, with a long discussion about everything that they've noticed about their pet's behaviour and habits. Secondly, a careful physical examination has to be conducted, using instruments like the stethoscope and thermometer to learn more about the inner workings of the body. Thirdly, extra tests need to be carried out, including laboratory work (e.g. blood and urine tests), diagnostic imaging (x-rays, ultrasound, MRI scans and endoscopy) and other work ups (such as ECGs and biopsies). When these three stages have been completed, it's usually possible to make a diagnosis of the cause of the pet's problem, and a treatment plan can be put into place.
In Bella's case, despite carrying out a thorough investigation, I could find no specific cause of the decline in her enthusiasm for life. Physically, she was in excellent condition. Laboratory work showed a small elevation in her liver enzymes, suggesting minor liver disease, but nothing more. X-rays revealed no evidence of serious abnormalities in her chest or abdomen. She was still in reasonable form, but just not thriving, and her owner was not sure what to do next. I explained that we could proceed further with investigations, including a detailed ultrasound inspection of her liver, an MRI scan, and perhaps even taking a biopsy. Her owner was unwilling to go this far, for two reasons. First, he didn't want to put Bella through any more procedures that she might find stressful. And secondly, he didn't feel that he could afford the extra tests that I was suggesting. On reflection, having considered the options, he decided that he had done enough, and he chose to take a "wait and see" approach. I gave Bella medication for the common, treatable causes of elevated liver enzymes. The plan was to check Bella once a month, and if she showed signs of physical deterioration, we could take action as needed.
As it happened, three weeks after Bella's initial work up, she suffered a sudden dramatic deterioration. One morning, she refused to get out of her bed. Her owner had to carry her into the car to bring her to my clinic. When I examined her, her gums, which had been a healthy pink colour, were almost white, and her heart, which had been slow and steady, was racing. An ultrasound examination confirmed my suspicions. Bella was suffering from internal bleeding, and the source was a small tumour, the size of a plum, on her liver. As is often the case, the surface of this tumour must have ruptured, like the skin of a ripe plum splitting. Once this had happened, blood had started to leak into her abdomen, causing the current crisis.
It was very likely that this was a malignant liver tumour, and after discussions, Bella's owner agreed that she had reached the end of her enjoyable life. While, in theory, it was possible to operate on her in an attempt to remove the tumour, the chances of a full recovery were small. Euthanasia, to make the end easy and painless for Bella, was the only kind option.
Bella's remains were sent for cremation, and three weeks later, when her owner came to collect her ashes, he asked me for a quick word. The death of a pet is often a major trauma in an owner's life, and for vets, one of our important roles is to be as supportive as possible. I asked Bella's owner into a private consulting room, and we sat down together to have a chat.
He had a simple question for me: "I know that you did your best for Bella, but one thing has been troubling me. Why didn't her blood tests tell you that she had cancer?"
This is one of the common misconceptions held by owners. The truth is that there is no blood test that is able to make a diagnosis of "cancer". Laboratory work, including blood tests, is able to give all sorts of information about the inner workings of the body, but it cannot tell the difference between cancer and other diseases. I knew from the changes in Bella's liver enzymes that there was something abnormal happening in her liver, but there is a long list of common, relatively benign, diseases that can cause such an elevation of liver enzymes. Without doing the extra tests that we had discussed (e.g. ultrasound), it was impossible to diagnose the cancer. And in any case, even if we had made the diagnosis, there was nothing we could have done to save her.
Bella's owner was relieved to hear my explanation: he had been worried that we had let her down in some way by not making an earlier diagnosis.
As it happens, a single blood test for all cancers may now be in the pipeline. News reports last week claim that an innovative test may soon be able to identify cancer-specific gene mutations in the DNA of a blood sample. Like all new technologies, it won't be cheap, but it may help future cases like Bella, It won't cure cancer, and the sad outcome will still be the same, but at least people like Bella's owner may be better prepared for that difficult day when they need to say their final goodbye.