A symptom on its own rarely points to one diagnosis. Tiredness, for instance, is common to anaemia, an underactive thyroid, poorly controlled diabetes, sleep apnoea, depression and simply not sleeping enough. What a doctor does is narrow that list using your history and examination first, then order blood tests to confirm or rule out the few possibilities that remain plausible. The tests come last, not first.

That ordering matters for how you read these pages. Each one lists the tests commonly requested for a symptom, but seeing a test listed does not mean you need it. A 25-year-old with tiredness and heavy periods gets a different panel from a 70-year-old with tiredness and weight loss, even though the presenting complaint is worded the same way.

Using these pages before an appointment

The most practical use is preparation. Appointments are short, and the details a doctor needs are often the ones patients forget to mention. Before you go, it helps to have thought about:

Duration and pattern — when it started, whether it is constant or comes and goes, and whether anything reliably makes it better or worse.
What changed around the same time — new medicines, a new job, a change in diet, weight loss or gain, a recent infection, or a period of unusual stress.
Associated symptoms you might not connect — fever, night sweats, appetite change, bowel or bladder changes, rashes. These are often what shifts a diagnosis.
Your medicine list, including things you buy yourself — supplements, painkillers and herbal products all affect blood results and are frequently left out.

Symptoms that should not wait

Some presentations need same-day assessment rather than a blood test booked for next week. Crushing or spreading chest pain, sudden breathlessness, one-sided weakness or facial droop, the worst headache of your life, coughing or vomiting blood, a fever with confusion or a rash that does not fade under pressure, and any bleeding that will not stop all fall into this group. If something in that list applies, contact emergency services rather than reading further.

Medical disclaimer: This page is for general educational purposes only and does not constitute medical advice. Reference ranges vary between laboratories, and results must be interpreted alongside your symptoms and history. Always consult a qualified healthcare professional for diagnosis and treatment. Read our full disclaimer.