Pregnancy tests are increasingly used to diagnose, or rule out, testicular cancer as the illness produces the same hCG hormone that is produced by the developing placenta.
Teenager Byron Geldard, from Huntingdon, Cambridgeshire, first went to the doctor after finishing his exams, complaining of a pain in his side, But he was told by his GP that it was just 'muscle soreness' caused by exercise at the gym.
But after he and his friends returned from a boys' holiday to Kavos, he went back to the doctor, who found a lump in his side and immediately referred him for an ultrasound.
Scans revealed that he had a tumour that had spread to his lungs. Byron was told he had cancer but that doctors were unsure which type it was.
The radiologist at Hinchingbrooke Hospital told him that Addenbrooke's Hospital, in Cambridge, would be in touch.
He returned home to find the phone ringing and Addenbrooke's on the other end, telling him they had a bed on the Teenage Cancer Trust ward.
Byron said: "They didn't know what type of cancer I had. I could have had four or five different types.
"The doctor kept saying things but it wasn't really going in.
"I left the room and fainted - I think it was the fear of the unknown."
He was referred to the Teenage Cancer Trust unit in Cambridge, which asked him to provide a sample for a pregnancy test to check his hormones.
After getting a positive result, Byron was diagnosed with stage four testicular cancer which had spread to his abdomen and lungs.
It was the day before his A-level results were released, which, despite months of worry, immediately became insignificant.
Byron said it was difficult to come to terms with the news and that it took him a while to accept what was happening.
He said: "It was all very surreal to be honest.
"There I was with a positive pregnancy test and something growing inside of me. I thought I was going to end up in a documentary."
He added: "I gave the hospital a urine sample, the pregnancy test came back positive, and I started chemotherapy the day after I was diagnosed.
"I would go in for five days in a row and have the chemo constantly.
"It really took it out of me - my brain was muddled and I found it hard to concentrate on long films or books.
"Throughout my cycles of chemo the hospital would monitor the pregnancy hormone through blood tests.
"The hormone had been really, really high, but it was gradually declining which was great news, and the tumours in my lungs and abdomen were shrinking."
In December, Byron had surgery to remove the growth in his abdomen, a testicle, and the lymphatic system behind his stomach. And on January 19, tests revealed that he was cancer free.
He said: "It was a really strange feeling to be told I was ok again - you're just sort of expected to go back to normal, but my mindset has completely changed.
"Before all this happened you think your life is pretty much guaranteed until you're about 85 but it gave me the realisation of my own mortality - I sort of had an early midlife crisis.
Now aged 19, Byron has become an ambassador for the Teenage Cancer Trust and is hoping to spread awareness.
He has enlisted the help of comedian John Bishop for a YouTube video about his experiences, plans to visit local schools and is also writing a stand up comedy routine about testicular cancer to help educate people.
Byron said: "Cancer is no longer a death sentence and I would like to spread that message."
The Teenage Cancer Trust said pregnancy tests had been used to diagnose testicular cancer for around six years.
A spokeswoman said they were considered reliable as the same proteins are detected in the urine of testicular cancer patients and of pregnant women.