The goals of testing are to detect high blood pressure, confirm that it is persistent over time, find out whether it is being caused by a particular disease that could be treated, check the health of various body organs, get a baseline prior to starting treatment, and monitor blood pressure and organ health during the period of treatment.
Laboratory tests cannot diagnose hypertension, but tests are frequently requested to help evaluate and monitor organ function and specific tests are sometimes requested to detect diseases that may be causing the high blood pressure or making it worse.
General tests that may be requested include:
- Urinalysis - to help assess kidney function
- Haematocrit – as part of a full blood count (FBC) to evaluate the ratio of fluid to solids in the blood
- Urea and creatinine – to detect and monitor kidney disease or to monitor the effect of drug treatment on the kidneys
- Electrolytes – sodium and potassium – some high blood pressure treatments can cause high sodium and potassium loss
- Fasting glucose – to determine if blood glucose levels are normal
- Calcium – increased activity of the parathyroid glands produces an increase in serum calcium which is associated with high blood pressure
- Lipid profile – to check levels of total cholesterol, HDL cholesterol, LDL cholesterol and triglycerides because persistent hypertension promotes hardening of the arteries (atherosclerosis)
Specific tests that may be requested because of the patient’s medical history, physical findings or general laboratory test results to help detect, diagnose and monitor conditions causing secondary hypertension include:
- Aldosterone and renin – to help detect the overproduction of aldosterone by the adrenal glands (which may be due to a tumour)
- Cortisol – to detect an overproduction of cortisol that may be due to Cushing’s syndrome
- Catecholamines and methylated amines – adrenaline, noradrenaline and their metabolites are used to help detect the presence of a phaeochromocytoma (a tumour of the adrenal gland) that can cause episodes of severe hypertension
- Parathyroid hormone (PTH) – if calcium is found to be increased
- Thyroid stimulating hormone (TSH) and free T4 – to detect and monitor thyroid dysfunction
Blood pressure measurement
This is the primary tool for detecting and monitoring hypertension. Although it can now be evaluated with a variety of electronic devices, blood pressure is traditionally and most accurately measured with a stethoscope and a blood pressure cuff (a sphygmomanometer – which includes a cuff, a bulb, and a pressure dial that reads the pressure in millimetres of mercury (mm Hg)). The cuff is placed on a patient’s upper arm and a bulb attached to the cuff is squeezed and released several times to inflate the cuff and increase pressure on the arm until the arterial blood flow is temporarily shut off.
The person taking the blood pressure listens through the stethoscope (which has been placed over the artery in the patient’s arm) while slowly releasing the air and reducing the pressure in the cuff. The pressure at which the heartbeat can be heard again is the systolic pressure. The pressure at which the sound again disappears is the diastolic pressure. The pressure is given as systolic over diastolic; for instance, 120 over 80 is a systolic pressure of 120 and a diastolic of 80 mm Hg.
Blood pressure measurements are usually performed with the patient sitting quietly for a few minutes but may also be done in other postures, such as standing. If a patient has an elevated blood pressure, the pressure in the other arm may be measured to confirm the finding. Since blood pressure can and will vary, a diagnosis of hypertension is not made from a single measurement, but will involve multiple measurements made at different times. It is not a single high reading that the doctor is interested in, but persistent high blood pressure.
The doctor may ask the patient to wear a device that monitors and records the blood pressure at regular intervals during the day to monitor it over time. This is especially helpful during the diagnostic process and can help rule out the high measurements that only occur when the patient is in the doctor’s surgery. This is known as the ‘white coat phenomenon,’ which has been estimated to account for as much as 10-20% of suspected cases of hypertension. There are now electronic blood pressure measuring devices that can be used in the home. These can be used effectively but should be checked at intervals against the findings at the doctor’s surgery to ensure accuracy.
These forms of blood pressure measurement are considered indirect. Very rarely, a direct measurement of blood pressure may be required. This can be obtained by inserting a catheter into an artery to measure the pressure inside the blood vessel.
As part of the diagnostic process and to help evaluate the status of vital organs, the doctor may request or perform one or more of the following:
- ECG (electrocardiogram) – to evaluate the heart rate and function
- Eye examination – to look at the retina for changes in the appearance of the blood vessels (retinopathy)
- Physical examination – to help evaluate the kidneys, to look for abdominal tenderness, to listen for bruits (the sound of blood flowing through a narrowed artery), to examine the thyroid gland in the throat for any enlargement or signs of dysfunction, and to detect any other clinical signs as they present
- Imaging scans, such as X-ray or ultrasound of the kidneys or X-ray of the chest.