How doctors diagnose illness: history, examination and tests

A diagnosis is the name of the condition that explains your symptoms. People often imagine that doctors reach it with a single clever test, but in modern medicine (sometimes called allopathy) it is usually a step-by-step process. This article explains those steps using information from MedlinePlus, the US Food and Drug Administration (FDA) and the National Health Service (NHS) of the United Kingdom. It is general information and cannot replace a visit to a doctor.

1. Introduction

Diagnosis is a form of detective work. Your doctor collects clues, lists the possible explanations and then uses tests to decide which explanation fits best. MedlinePlus describes the order as history, physical exam, a list of possible causes and then tests ^[1]. Understanding this order helps you give useful information and ask good questions.

2. Step one: your history

The first and often most useful clue is your own story. MedlinePlus says a provider usually asks about your symptoms, including when they began, what seems to trigger them and how they affect daily life. The provider also asks about past medical problems, family health history, and any changes in sleep, diet, activity, work, mood or relationships ^[1].

Medicines matter too. MedlinePlus notes that prescription medicines, over-the-counter medicines and supplements can themselves cause symptoms, so you should tell your doctor about everything you take ^{[1, 2]}. Writing a short list before your visit makes this easier.

3. Step two: the physical examination

Next the doctor examines you, for example by listening to your heart and lungs, feeling the abdomen or checking reflexes. According to MedlinePlus, what the doctor finds on examination can make some conditions more likely and others less likely ^[1]. The examination does not give a final answer by itself, but it points the doctor towards the most useful tests.

4. Step three: a list of possible causes

After the history and examination, the doctor drafts a list of conditions that could explain your symptoms. This list is called a differential diagnosis. The condition that becomes clear after other possibilities are ruled out is called the final diagnosis ^[1].

A medical textbook chapter on evidence-based medicine from the US National Library of Medicine explains that diagnosis relies on probability. How much a test result tells you depends on how likely the condition was before the test, which doctors call the pretest probability ^[5]. This is why a test that makes sense for one person may not be useful for another.

5. Step four: choosing tests

Tests are used to confirm or exclude items on the list. MedlinePlus lists blood or urine tests, biopsies (small samples of tissue examined under a microscope), imaging such as X-ray, ultrasound or MRI, and mental health screening. Tests for any urgent possibility are done first ^[1].

The FDA explains that laboratory tests are used for several purposes ^[3]:

• To detect health changes before symptoms appear.
• To help diagnose a disease or condition.
• To plan treatment.
• To check how well a treatment is working.
• To follow a condition over time.

Common blood tests

The NHS lists some of the commonly used blood tests ^[4]:

• A full blood count, which counts blood cells and can show iron-deficiency anaemia (a shortage of healthy red blood cells caused by low iron).
• A blood glucose test, which measures blood sugar and is used for diabetes checks.
• A lipid profile, which measures cholesterol and relates to heart disease and stroke risk.
• A thyroid stimulating hormone test, which checks how well the thyroid gland works.

For most blood tests, blood is drawn from the inner elbow and the visit takes about 10 minutes. You may feel a brief scratch, and bruising can appear but usually fades within a few days. Your clinician will tell you if you need to fast or stop certain medicines before the test ^[4].

6. Understanding your results

Reference ranges

A reference range is the span of results considered typical, based on large groups of healthy people. MedlinePlus advises comparing your result only with the range printed on your own lab report, because different labs use different methods and units, so their ranges differ ^[2]. Trends are easiest to follow when the same lab does your tests ^[2].

Abnormal does not always mean disease

MedlinePlus explains that a result outside the range may or may not signal a health problem, and healthy people sometimes fall outside it. A result inside the range also does not guarantee good health, especially if you have symptoms ^[2]. Food, drink, medicines, hard exercise and menstrual periods can all affect results ^[2]. The FDA adds that age, sex, medical history and differences between laboratories matter too ^[3].

False results

A false positive shows a condition that is not present, and a false negative misses one that is. MedlinePlus says both are uncommon but possible, and a provider may repeat a test if a result does not seem to fit ^[2]. A result described as inconclusive means it is unclear and usually needs repeating ^[2].

Waiting for results

The NHS says blood test results usually take a few days and sometimes up to a few weeks. If you have not heard back after a few weeks, contact the surgery or specialist, and if the results are confusing, ask a health professional to explain them ^[4].

7. Taking part in your own diagnosis

• Bring a list of your symptoms, medicines and supplements, and your family history.
• Ask what each test is looking for and what happens next if it is normal or abnormal.
• Do not rely on home test kits instead of tests your doctor orders. MedlinePlus says you can ask your provider or pharmacist which home tests are trustworthy ^[2].
• If you remain unsure about a diagnosis, you can ask for a second opinion, which is covered in another article in this library.

8. When to see a doctor

See a doctor if symptoms are new, last longer than expected, keep returning or make daily life harder. Contact your doctor or clinic if test results have not reached you or if you do not understand them ^[4]. Never delay care because a home test looked normal, since a normal result does not rule out a problem ^[2].

Sudden, severe or fast-worsening symptoms need urgent help and should not wait for test results. In an emergency, call your local emergency number (112 in India, 911 in the United States).

9. Conclusion

Diagnosis in modern medicine is a process that combines your history, an examination, a list of possible causes and carefully chosen tests. No single result tells the whole story, so your doctor reads results together with your symptoms and history. You help most by giving a complete history, naming every medicine you use and asking what results mean for you. Your doctor decides on any treatment.