Millions of people worldwide are struggling with chronic fatigue syndrome. But the disease has been so confusing and mysterious for decades, we still can't even diagnose it properly, let alone find a reliable treatment or cure.

But now scientists have tracked down a bunch of biomarkers which correlate with the syndrome. And that's huge, because it means a blood test for reliably diagnosing the disease is finally on the horizon.

Chronic fatigue syndrome (also called myalgic encephalomyelitis, ME/CFS) is one of the most baffling medical conditions out there. Its main symptom is persistent, unexplained fatigue that can last for months or years, and can be so disabling people literally can't do much else but lie in bed.

More than one million Americans and some 2.6 percent of the global population struggle with ME/CFS, but the wide range of vague symptoms it causes has made it extremely difficult to study, even leading to the false thinking that it's a psychological issue, not a 'real disease'.

But people with ME/CFS do tend to have fluctuating flu-like symptoms and body aches which have prompted scientists to wonder about a possible link with the immune system or inflammation.

What's made diagnostics especially tricky is the perplexing finding that the typical biomarkers doctors use to find inflammation are not always elevated in ME/CFS patients. Bloodwork tends to come back with conflicting results, getting us no closer to a proper diagnostic test.

But there are around a 100 biomarkers that scientists can hunt for, especially if they look at molecules that are involved in controlling inflammation. These markers are called cytokines -  small proteins involved in signalling of the immune system.

With that in mind, researchers from Stanford University School of Medicine went hunting for a more comprehensive bloodwork profile and hit jackpot - a whopping 17 cytokines that correlate with the severity of chronic fatigue symptoms.

The team scanned blood samples from 186 people with ME/CFS and compared them with blood from 388 healthy people, looking specifically at the levels of 51 different cytokines that slosh around in the blood.

Only one of these cytokines - tumour growth factor beta - turned out to have consistently higher levels in all ME/CFS patients in comparison to controls. But when the researchers analysed data of disease severity and duration, they found that 17 cytokines could be used to track the condition.

In people who had had the syndrome for longer and with more severe symptoms, more of these cytokines were sloshing around the bloodstream.

Out of those 17 molecules the researchers identified, 13 are known to promote inflammation, so that's more points scored for the hypothesis that chronic fatigue is an inflammatory disease - just one we haven't properly diagnosed yet.

"There's been a great deal of controversy and confusion surrounding ME/CFS - even whether it is an actual disease," says senior researcher Mark Davis from Stanford University.

"Our findings show clearly that it's an inflammatory disease and provide a solid basis for a diagnostic blood test."

Now, this particular study only showed a correlation, so we don't yet know if those elevated cytokines are contributing to the condition or are simply there because people have the syndrome.

But at least it's a clear biological marker that will help put to rest the many myths surrounding chronic fatigue.

"This is a field that has been full of scepticism and misconception, where patients have been viewed to have invented their disease," lead researcher Jose Montoya from Stanford told NPR News.

"These data clearly show the contrary, and demonstrate what can be achieved when we couple good research design with new technology."

And it's not just a blood test that ME/CFS patients can finally look forward to now. The researchers also say that their findings offer up potential to test immunomodulatory drugs as a new treatment for the condition.

These recent findings are now joining the pool of other recent studies that have found biological markers for chronic fatigue. Just last year, researchers discovered metabolic abnormalities in people with this syndrome.

And earlier this year a new study showed that people with ME/CFS also have abnormal levels of specific gut bacteria.

All this work is really moving us away from thinking about this serious condition as an 'imaginary' disease. Let's hope scientists can also track down a treatment soon.

The study was published in PNAS.