Blood tests for dizziness: what a GP may check and what they cannot show

Dizziness is a symptom rather than a diagnosis. The NHS uses the word to include feeling off-balance, giddy, lightheaded or faint, or feeling that you or things around you are spinning (vertigo). Because the causes include the inner ear, blood pressure, the heart, medicines, anxiety and migraine, most of which do not show up in a blood sample, a blood test is rarely the main investigation.

A South West London ICB dizziness pathway for GPs lists the investigations to consider as checking blood glucose, postural blood pressure, bloods for anaemia and an ECG. Those are the tests that fit the common treatable causes of feeling lightheaded, and they are usually done alongside an examination of the ears, eyes and nervous system.

In other words, if you have had a dizzy spell and are wondering what blood tests you will get, the realistic answer is a small number, chosen from your history. Many people with dizziness have no blood test at all, because the history and examination point clearly to the cause.

The sources read for this guide describe two blood-based checks as the usual ones, and a few related checks that are not blood tests.

The NHS says that for iron deficiency anaemia the GP will usually do a full blood count to find out whether the number of red blood cells is normal, and that this test also looks at other types of cells and checks for other types of anaemia such as vitamin B12 or folate deficiency. You do not need to do anything to prepare. If the reason for the anaemia is not clear, the GP may arrange other tests, such as a faecal immunochemical test (FIT), or refer you to a specialist.

For blood sugar, the NHS describes low blood sugar as usually affecting people with diabetes who take insulin or some other diabetes medicines and as rare in people without diabetes. A finger-prick or laboratory glucose measurement can show whether the sugar level was low or raised. If a dizzy spell happened some time ago, the test can only show the level at the time of sampling, so a normal result does not rule out an earlier low.

The NHS and nidirect list the common causes of dizziness as inner-ear problems such as labyrinthitis, migraine, stress or anxiety, postural hypotension, dehydration or heat exhaustion, and ear conditions. Most of these are recognised from the history and examination rather than from blood.

nidirect says dizziness that happens when you move your head, fits the pattern of vertigo and lasts for several days is often caused by a viral ear infection. It also says dizziness occurring when you are standing upright is probably not related to the ear. Blood tests will not distinguish these, which is why a clinician asks when the dizziness happens and for how long.

The South West London pathway describes bedside examinations that do the diagnostic work for positional vertigo, including the head impulse test, nystagmus assessment and positional manoeuvre, and says it supports referral to ENT or audiovestibular services if there is no improvement. These are not laboratory tests.

nidirect suggests keeping a diary of your dizziness and taking it to your GP appointment, noting what you were doing, how long it lasted and how bad it was, and whether you had other symptoms such as fainting, vomiting, nausea, blurred vision, headache, hearing loss or tinnitus. The GP will want to know exactly what you mean by dizziness, whether it started for no obvious reason or followed an illness, whether you have repeated episodes and how long it lasts.

If you are taking prescription medicine, nidirect says your GP will probably review it to check whether dizziness is a possible side effect and, if necessary, prescribe a different medication. That review often tells more than a blood test.

The NHS says to see a GP if dizziness will not go away or keeps coming back, or if you are finding it harder to hear or speak, have ringing in the ears, have double vision or blurred vision, have numb or weak face, arms or legs, or have other symptoms such as changes to your pulse, fainting or collapsing, headaches, or feeling or being sick.

The South West London pathway lists symptoms that warrant urgent assessment for stroke, including acute continuous vertigo with a new headache, severe headache or neck pain, being unable to stand or sit up unaided, focal weakness or altered sensation of the face or limbs, difficulty with speech, swallowing or double vision, and recent trauma. Sudden onset deafness is a reason for urgent ENT referral. Those are not situations in which to wait for a routine blood test.