Janet was 44 when she came to see me, worn out by months of worsening tiredness, aching joints, low mood and a brain fog so bad it was affecting her at work. 

She was convinced she was perimenopausal. What else could it be?

But something was off. Her periods were still arriving like clockwork – not a day's change to her cycle. She had also lost weight without trying. Neither of those things fits perimenopause.

So I ran some blood tests. These picked up abnormal markers in her immune system – something wasn't right. And after I referred her to a specialist, Janet received a diagnosis she never expected: lupus.

It's hardly surprising that Janet was taken aback by the diagnosis. After all, she, like so many others, had never heard of lupus.

But it's a condition that affects a surprising number of women – and some are wrongly told their symptoms are due to the menopause, depriving them of crucial, life-improving treatment.

Lupus is an autoimmune condition. That means the body's immune system, which is supposed to protect you from infection, mistakenly turns on healthy tissue instead – attacking joints, skin, kidneys, the heart, even the brain.

The cause of lupus is still unclear, but there is some evidence that it could be triggered by environmental factors such as infections, certain medicines, smoking and UV light exposure.

It is a serious condition that affects around 70,000 people in the UK – roughly one in 1,000 – but too often it goes unrecognised for years.

Research published last year by Swansea University found the average wait from first symptoms to diagnosis is seven and a half years. The researchers found that some patients wait up to 40 years.

One woman even described going to her GP with swollen legs and being told she had anxiety. Another spent 15 years being told she had stress, postnatal depression and menopause – before anyone thought to test for lupus.

This is not rare. Studies suggest up to two-thirds of patients receive at least one wrong diagnosis before the right one.

Part of the problem is that lupus and perimenopause look remarkably alike. Fatigue, joint pain, brain fog, mood changes, poor sleep – all are symptoms of both.

Lupus is even known as 'the great imitator' because it can so convincingly resemble other conditions. It is far more common in women than men – 90 per cent of diagnoses are amongst females.

It also typically strikes between the ages of 15 and 55 – meaning many women will develop it right at the point when perimenopause starts.

The result is that many GPs – understandably – reach for the most common explanation first.

Perimenopause, the early stages of the menopause, is far more prevalent than lupus, and generally, GPs do not use blood tests to diagnose it – particularly if women are aged over 45.

Often, family doctors will then prescribe hormone replacement therapy, also known as HRT, a highly effective menopause medication that can ease symptoms.

In most cases, this is the correct approach. Lupus is, after all, relatively rare.

But it is important that GPs and patients alike know how to spot the difference between the two conditions.

For one thing, lupus isn't treated with HRT, so symptoms may not improve with it. 

There are telltale signs of the disease to look out for, however. These include a rash across the cheeks and nose that gets worse in sunlight, mouth ulcers, and unexplained low-grade fevers.

Patients who experience these symptoms could ask their GP to be tested for lupus.

The reason that a diagnosis is so important is that there are effective treatments. Lupus cannot be cured but the symptoms can be eased.

Mild cases are often managed with anti-inflammatory drugs and hydroxychloroquine, an antimalarial that damps down the immune system's overactivity and has been used to treat lupus for decades.

More severe cases may need steroids, immunosuppressants, or newer targeted drugs. 

Janet was started on steroids and saw a marked improvement in her symptoms. Her joint pain eased and her brain fog lifted. She's often still fatigued but she's a world away from where she was when she first came to my surgery.

And the treatment picture for patients like Janet is improving.

Earlier this month, results from a major international trial published in The Lancet medical journal, showed that a new oral pill called enpatoran produced significant benefits in patients with moderate to severe lupus.

The news sparked excitement amongst doctors and lupus patients around the world.

The drug works by blocking the immune signals that drive lupus flares. More than half of patients on it responded to treatment, compared with around a third on a placebo. It is now moving into the final stage of clinical trials before it can be considered for approval.

None of this means that every woman in her 40s with tiredness and joint pain has lupus – most don't.

But Janet's story is a reminder that perimenopause and menopause cannot – and should not – be the explanation for every health issue that affects women in their 40s and 50s.