Sometimes, getting a diagnosis when you're unwell is simple: you make an appointment, see a doctor, possibly have a straightforward test, then wait for the results - a nd the vast majority of the time, patients are correctly diagnosed by medical professionals.

But this isn't always the case.

Some conditions have symptoms that mimic other health problems, or which can be very vague and inconsistent. Also, there aren't always straightforward tests available, which can make determining a diagnosis far more complex - or, if a condition is especially rare, it might only be considered once everything else has been ruled out.

Here are six of the illnesses that can be harder to spot in initial medical consultations...

CERTAIN CANCERS

A 2011 Rarer Cancers Foundation study found that a quarter of cancer patients were only diagnosed once the disease had spread, with many having been initially sent home by their GPs, who dismissed the symptoms as something less serious - although misdiagnosed cancers often include the more common types, like breast, colorectal, lung and pancreatic cancer. These might be misdiagnosed because symptoms often mimic other, non-cancerous conditions - inflammatory breast cancer, for example, has similar symptoms to mastitis; colorectal cancer may be mistaken for irritable bowel syndrome (IBS) or ulcerative colitis; lung cancer symptoms can be similar to those caused by pneumonia, bronchitis and tuberculosis; and pancreatic cancer can be confused with inflammatory bowel diseases, pancreatitis and diabetes.

PARKINSON'S DISEASE

The symptoms of this degenerative disorder of the central nervous system include tremors, stiff muscles and problems with balance or walking. It can be mistaken for Alzheimer's disease, stroke, stress, or even a head injury, as there are no simple lab tests that can diagnose Parkinson's.

COELIAC DISEASE

Caused by an inability to digest gluten, which can start suddenly, this digestive disorder can result in abdominal pain, headaches, joint pain, itchy skin, diarrhoea, vomiting and weight loss, although sometimes symptoms can be mild. While a blood test can diagnose coeliac disease, it's often mistaken for other conditions, including irritable bowel syndrome (IBS), diverticulitis, intestinal infections and chronic fatigue syndrome, because the symptoms can be very similar and, in some cases, identical. However, correctly diagnosing coeliac disease is crucial as, unlike conditions like IBS, if a strict gluten-free diet isn't followed, serious long-term complications can occur.

LUPUS

Fatigue, rashes and joint pain are some of the common symptoms of this autoimmune disease, that can affect almost any part of the body, including organs like the heart and kidneys, in severe cases. Although blood and urine tests can indicate lupus, such tests are not always conclusive, and it's sometimes misdiagnosed as things like arthritis or fibromyalgia, or even dismissed entirely for years.

HYPOTHYROIDISM

It's thought that around 3% of Brits have an underactive thyroid, known as hypothyroidism, which can be diagnosed with a blood test and treated with a synthetic hormone. But it can remain undiagnosed for years because symptoms - which include tiredness, weight changes, forgetfulness and lack of concentration - can be very subtle, or might not be recognised as being linked to one underlying cause. Sufferers may be told there's nothing wrong, or that they're depressed.

MULTIPLE SCLEROSIS

Multiple sclerosis, commonly known as MS, is an autoimmune disease that attacks the central nervous system, resulting in, among other things, problems with muscle movement, balance and vision. There are different types but often it's progressive, and symptoms - such as muscle spasms, impaired coordination, blurred vision, numbness and tingling - may be quite mild and vague for some time, before getting worse. There's no single test to diagnose MS, and it can sometimes be misdiagnosed as a viral infection or lupus, or even Alzheimer's and bipolar disorder. Blood tests can be helpful in ruling out some other disorders though, and tests, including an MRI scan, may indicate MS.

:: Don't be too alarmed

While it's important to highlight conditions that may be missed, it's also important to remember that it's not always the case that symptoms like those outlined above actually indicate something more serious.

Also, Dr Helen Stokes-Lampard, spokesperson for the Royal College of GPs, points out that GPs have to make decisions based on a single 10-minute consultation. "It goes without saying that some conditions are more difficult to diagnose than others," she says. "Many have symptoms that are very similar to others, and when one condition is more common than the other, it makes sense that it's considered the most likely possibility."

She stresses that lists of hard-to-diagnose conditions can worry patients, and break down the trust between them and their GP. "It's essential that patients aren't scared by lists that over-exaggerate misdiagnosis, particularly when life-threatening and debilitating diseases such as cancer and Parkinson's are included."

Dr Stokes-Lampard notes that during more than 10 years of training, family doctors become familiar with a diverse curriculum and will be aware of the different conditions that symptoms could indicate, even if they're the less likely diagnosis. And when fully-trained, GPs have ongoing appraisals and continued professional development. "The College works to develop new learning tools for GPs at every stage in their careers, and topics include some of the most difficult conditions to diagnose, such as coeliac disease, and cancer," she adds.