For most patients, health care providers diagnose high blood pressure when blood pressure readings are consistently 140/90 mmHg or above. A blood pressure test can be done in a health care provider’s office or clinic. To track blood pressure readings over a period of time, the health care provider may ask the patient to come into the office on different days and at different times. The health care provider also may ask the patient to check readings at home or at other locations that have blood pressure equipment and to keep a written log of results. The health care provider usually takes 2–3 readings at several medical appointments to diagnose high blood pressure. Using the results of the blood pressure test, the health care provider will diagnose prehypertension or high blood pressure if:

- For an adult, systolic or diastolic readings are consistently higher than 120/80 mmHg.

- A child’s blood pressure numbers are outside average numbers for children of the same age, gender, and height.

Once the health care provider determines the severity, he or she can order additional tests to determine if the blood pressure is due to other conditions or medicines or if there is primary high blood pressure. Health care providers can use this information to develop a treatment plan.

Hypertension is diagnosed on the basis of a persistently high resting blood pressure. Traditionally, the National Institute of Clinical Excellence recommends three separate resting sphygmomanometer measurements at monthly intervals. The American Heart Association recommends at least three resting measurements on at least two separate health care visits.

For an accurate diagnosis of hypertension to be made, it is essential for proper blood pressure measurement technique to be used. Improper measurement of blood pressure is common and can change the blood pressure reading by up to 10 mmHg, which can lead to misdiagnosis and misclassification of hypertension. Correct blood pressure measurement technique involves several steps. Proper blood pressure measurement requires the person whose blood pressure is being measured to sit quietly for at least five minutes which is then followed by application of a properly fitted blood pressure cuff to a bare upper arm. The person should be seated with their back supported, feet flat on the floor, and with their legs uncrossed. The person whose blood pressure is being measured should avoid talking or moving during this process. The arm being measured should be supported on a flat surface at the level of the heart. Blood pressure measurement should be done in a quiet room so the medical professional checking the blood pressure can hear the Korotkoff sounds while listening to the brachial artery with a stethoscope for accurate blood pressure measurements. The blood pressure cuff should be deflated slowly (2-3 mmHg per second) while listening for the Korotkoff sounds. The bladder should be emptied before a person's blood pressure is measured since this can increase blood pressure by up to 15/10 mmHg. Multiple blood pressure readings (at least two) spaced 1-2 minutes apart should be obtained to ensure accuracy. Ambulatory blood pressure monitoring over 12 to 24 hours is the most accurate method to confirm the diagnosis.

An exception to this is those with very high blood pressure readings especially when there is poor organ function. Initial assessment of the hypertensive people should include a complete history and physical examination. With the availability of 24-hour ambulatory blood pressure monitors and home blood pressure machines, the importance of not wrongly diagnosing those who have white coat hypertension has led to a change in protocols. In the United Kingdom, current best practice is to follow up a single raised clinic reading with ambulatory measurement, or less ideally with home blood pressure monitoring over the course of 7 days. The United States Preventative Services Task Force also recommends getting measurements outside of the healthcare environment. Pseudohypertension in the elderly or noncompressibility artery syndrome may also require consideration. This condition is believed to be due to calcification of the arteries resulting in abnormally high blood pressure readings with a blood pressure cuff while intra arterial measurements of blood pressure are normal. Orthostatic hypertension is when blood pressure increases upon standing.

Once the diagnosis of hypertension has been made, healthcare providers should attempt to identify the underlying cause based on risk factors and other symptoms, if present. Secondary hypertension is more common in preadolescent children, with most cases caused by kidney disease. Primary or essential hypertension is more common in adolescents and has multiple risk factors, including obesity and a family history of hypertension. Laboratory tests can also be performed to identify possible causes of secondary hypertension, and to determine whether hypertension has caused damage to the heart, eyes, and kidneys. Additional tests for diabetes and high cholesterol levels are usually performed because these conditions are additional risk factors for the development of heart disease and may require treatment.

Serum creatinine is measured to assess for the presence of kidney disease, which can be either the cause or the result of hypertension. Serum creatinine alone may overestimate glomerular filtration rate and recent guidelines advocate the use of predictive equations such as the Modification of Diet in Renal Disease (MDRD) formula to estimate glomerular filtration rate (eGFR). eGFR can also provide a baseline measurement of kidney function that can be used to monitor for side effects of certain anti-hypertensive drugs on kidney function. Additionally, testing of urine samples for protein is used as a secondary indicator of kidney disease. Electrocardiogram (EKG/ECG) testing is done to check for evidence that the heart is under strain from high blood pressure. It may also show whether there is thickening of the heart muscle (left ventricular hypertrophy) or whether the heart has experienced a prior minor disturbance such as a silent heart attack. A chest X-ray or an echocardiogram may also be performed to look for signs of heart enlargement or damage to the heart.

In people aged 18 years or older hypertension is defined as a systolic or a diastolic blood pressure measurement consistently higher than an accepted normal value (this is above 129 or 139 mmHg systolic, 89 mmHg diastolic depending on the guideline). Other thresholds are used (135 mmHg systolic or 85 mmHg diastolic) if measurements are derived from 24-hour ambulatory or home monitoring. Recent international hypertension guidelines have also created categories below the hypertensive range to indicate a continuum of risk with higher blood pressures in the normal range. The "Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation and Treatment of High Blood Pressure" (JNC7) published in 2003 uses the term prehypertension for blood pressure in the range 120–139 mmHg systolic or 80–89 mmHg diastolic, while European Society of Hypertension Guidelines (2007) and British Hypertension Society (BHS) IV (2004) use optimal, normal and high normal categories to subdivide pressures below 140 mmHg systolic and 90 mmHg diastolic. Hypertension is also sub-classified: JNC7 distinguishes hypertension stage I, hypertension stage II, and isolated systolic hypertension. Isolated systolic hypertension refers to elevated systolic pressure with normal diastolic pressure and is common in the elderly. The ESH-ESC Guidelines (2007) The results also demonstrated a correlation of chronically low vitamin D levels with a higher chance of becoming hypertensive. Supplementation with vitamin D over 18 months in normotensive individuals with vitamin D deficiency did not significantly affect blood pressure.

Few women of childbearing age have high blood pressure, up to 11% develop hypertension of pregnancy. While generally benign, it may herald three complications of pregnancy: pre-eclampsia, HELLP syndrome and eclampsia. Follow-up and control with medication is therefore often necessary.

The diagnosis for renovascular hypertension is done by:

- Blood test (for renal function)

- Urinary test (tests for microalbuminuria)

- Serology (to exclude systemic lupus erythematosus )

- Lipid profile

- Urinalysis (to exclude presence of red blood cells)

The ABCDE mnemonic can be used to help determine a secondary cause of hypertension

- A: Accuracy, Apnea, Aldosteronism

- B: Bruits, Bad Kidney

- C: Catecholamines, Coarctation of the Aorta, Cushing's Syndrome

- D: Drugs, Diet

- E: Erythropoietin, Endocrine Disorders

Prognosis of individuals with renovascular hypertension is not easy to determine. Those with atherosclerotic renal artery disease have a high risk of mortality, furthermore those who also have renal dysfunction have a higher mortality risk.

However, the majority of renovascular diseases can be improved with surgery.

Regular physical exercise reduces blood pressure. The UK National Health Service advises 150 minutes (2 hours and 30 minutes) of moderate-intensity aerobic activity per week to help prevent hypertension.

In studies, white coat hypertension can be defined as the presence of a defined hypertensive average blood pressure in a clinic setting, although it isn't present when the patient is at home.

Diagnosis is made difficult as a result of the unreliable measures taken from the conventional methods of detection. These methods often involve an interface with health care professionals and frequently results are tarnished by a list of factors including variability in the individual’s blood pressure, technical inaccuracies, anxiety of the patient, recent ingestion of pressor substances, and talking, amongst many other factors. The most common measure of blood pressure is taken from a noninvasive instrument called a sphygmomanometer. "A survey showed that 96% of primary care physicians habitually use a cuff size too small," adding to the difficulty in making an informed diagnosis. For such reasons, white coat hypertension cannot be diagnosed with a standard clinical visit. It can be reduced (but not eliminated) with automated blood pressure measurements over 15 to 20 minutes in a quiet part of the office or clinic.

Patients with white coat hypertension do not exhibit the signs indicative of trepidation and their increased blood pressure is often not accompanied by tachycardia. This is supported by studies that repeatedly indicate that 15%–30% of those thought to have mild hypertension as a result of clinic or office recordings display normal blood pressure and no unusual response to pressure stimulus. These persons did not show any specific characteristics such as age that may be indicative of a higher susceptibility to white coat hypertension.

Ambulatory blood pressure monitoring and patient self-measurement using a home blood pressure monitoring device is being increasingly used to differentiate those with white coat hypertension or experiencing the white coat effect from those with chronic hypertension. This does not mean that these methods are without fault. Daytime ambulatory values, despite taking into account stresses of everyday life when taken during the patient's daily routine, are still susceptible to the effects of daily variables such as physical activity, stress and duration of sleep. Ambulatory monitoring has been found to be the more practical and reliable method in detecting patients with white coat hypertension and for the prediction of target organ damage. Even as such, the diagnosis and treatment of white coat hypertension remains controversial.

Recent studies showed that home blood pressure monitoring is as accurate as a 24-hour ambulatory monitoring in determining blood pressure levels. Researchers at the University of Turku, Finland studied 98 patients with untreated hypertension. They compared patients using a home blood pressure device and those wearing a 24-hour ambulatory monitor. Researcher Dr. Niiranen said that "home blood pressure measurement can be used effectively for guiding anti-hypertensive treatment". Dr. Stergiou added that home tracking of blood pressure "is more convenient and also less costly than ambulatory monitoring."

Use of breathing patterns has been proposed as a technique for identifying white coat hypertension.

In one Turkish study of 438 consecutive patients, 38% were normotensive, 43% had white coat hypertension, 2% had masked hypertension, and 15% had sustained hypertension. Even patients taking medication for sustained hypertension who are normotensive at home may exhibit white coat hypertension in the office setting.

In general, individuals with white coat hypertension have lower morbidity than patients with sustained hypertension, but higher morbidity than the clinically normotensive.

However, it should be remembered that all the established published trials on the consequences of high blood pressure and the benefits of treating are based on one-time measurement in clinical settings rather than the generally slightly lower readings obtained from ambulatory recordings.

The debate and conflicting ideas revolve around whether or not it would be feasible to treat white coat hypertension, as there still is no conclusive evidence that a temporary rise in blood pressure during office visits has an adverse effect on health.

In fact, many cross sectional studies have shown that "target-organ damage (as exemplified by left ventricular hypertrophy) is less in white-coat hypertensive patients than in sustained hypertensive patients even after the allowance has been made for differences in clinic pressure". Many believe that patients with "white coat" hypertension do not require even very small doses of antihypertensive therapy as it may result in hypotension, but must still be careful as patients may show signs of vascular changes and may eventually develop hypertension. Even patients with established hypertension that is well-controlled based on home blood pressure monitoring may experience elevated readings during office visits.

Based on these studies, treating to a systolic blood pressure of 140, as long as the diastolic blood pressure is 68 or more, seems safe. Corroborating this, a reanalysis of the SHEP data suggests allowing the diastolic to go below 70 may increase adverse effects.

A meta-analysis of individual patient data from randomized controlled trials found the lowest diastolic blood pressure for which cardiovascular outcomes improve is 85 mm Hg for untreated hypertensives and 80 mm Hg for treated hypertensives. The authors concluded "poor health conditions leading to low blood pressure and an increased risk for death probably explain the J-shaped curve". Interpreting the meta-analysis is difficult, but avoiding a diastolic blood pressure below 68–70 mm Hg seems reasonable because:

- The low value of 85 mm Hg for treated hypertensives in the meta-analysis is higher than the value of 68–70 mm Hg that is suggested by the two major randomized controlled trials of isolated systolic hypertension

- The two largest trials in the meta-analysis, Hypertension Detection and Follow-up Program (HDFP) and Medical Research Council trial in mild hypertension (MRC1) were predominantly middle-aged subjects, all of whom had diastolic hypertension before treatment.

- The independent contributions of diseases and factors other than hypertension versus effects of treatment are not clear in the meta-analysis.

A more contemporary meta-analysis by the Cochrane Hypertension group found no benefits in terms of reduced mortality or morbidity from treating patients to lower diastolic targets than 90–100 mmHg.

Persistently increased blood pressure may also be due to kidney disease or hyperthyroidism. When a cause is not readily apparent, and especially when hypokalemia is identified, hyperaldosteronism should be considered. Diagnostic imaging, usually beginning with abdominal ultrasound, may identify that one or both adrenal glands are enlarged. Imaging may also detect metastasis and usually includes radiographs of the chest in addition to abdominal ultrasound and/or computerized tomography (CT).

The ratio of plasma aldosterone concentration (PAC) to plasma renin activity (PRA) can be used as a screening test for PHA. In cats with unilateral or bilateral zona glomerulosa tumors, the PAC may be very high while the PRA is completely suppressed. In cats with idiopathic bilateral nodular hyperplasia of the zona glomerulosa, the PAC may be slightly elevated or high normal. In the presence of hypokalemia even a mildly elevated aldosterone should be considered inappropriately high. A high-normal or elevated PAC with a low PRA indicates persistent aldosterone synthesis in the presence of little or no stimulation of the renin-angiotensin system.

The goal of treating systolic hypertension is to delay and reduce the extent of damage to the heart, the cerebrovascular system, and the kidneys. Lifestyle interventions are a crucial element of successful treatment, including a diet low in sodium (salt) and rich in whole grains, fruits, and vegetables. Clinical trials have also documented the beneficial effects of weight loss, increased physical activity, and limiting alcohol consumption.

In addition to lifestyle changes, medication can also be used to reduce systolic hypertension to safe levels, although medications frequently have side effects, often serious.

The diagnosis of renal artery stenosis can use many techniques to determine if the condition is present, a clinical prediction rule is available to guide diagnosis.

Among the diagnostic techniques are:

- Doppler ultrasound study of the kidneys

- refractory hypertension

- auscultation (with stethoscope) - bruit ("rushing" sound)

- captopril challenge test

- captopril test dose effect on the differential renal function as measured by MAG3 scan.

- renal artery arteriogram.

Ultrasonography (US) is the first-line imaging technique for the diagnosis and follow-up of portal hypertension because it is non-invasive, low-cost and can be performed on-site.

Signs of portal hypertension on ultrasound include dilatation of the portal vein of over 13 mm in diameter, a portal flow mean velocity of less than 12 cm/s on Doppler ultrasound, porto–systemic collateral veins (patent paraumbilical vein, spleno–renal collaterals and dilated left and short gastric veins), splenomegaly and signs of cirrhosis (including nodularity of the liver surface).

The hepatic venous pressure gradient (HVPG) measurement has been accepted as the gold standard for assessing the severity of portal hypertension. Portal hypertension is defined as HVPG greater than or equal to 5 mm Hg and is considered to be clinically significant when HVPG exceeds 10 to 12 mm Hg.

Increasing access to, and use of, genome profiling may provide opportunity for diagnosis based on presentation and genetic risk factors, by identifying ApoL1 gene variants on chromosome 22.

Unilateral primary hyperaldosteronism due to an adrenocortical adenoma or adrenocarcinoma can be potentially cured surgically. Unilateral adrenalectomy is the treatment of choice for unilateral PHA. Potential complications include hemorrhage and postoperative hypokalemia. With complete removal of the tumor, prognosis is excellent.

Bilateral primary hyperaldosteronism due to hyperplasia of the zona glomerulosa or metastasized adrenocortical adenocarcinoma should be treated medically. Medical therapy is aimed at normalizing blood pressure and plasma potassium concentration. Mineralocorticoid receptor blockers, such as spironolactone, coupled with potassium supplementation are the most commonly used treatments. Specific therapy for treating high blood pressure (e.g., amlodipine), should be added if necessary.

Primary hyperaldosteronism can be mimicked by Liddle syndrome, and by ingestion of licorice and other foods containing glycyrrhizin. In one case report, hypertension and quadriparesis resulted from intoxication with a non-alcoholic pastis (an anise-flavored aperitif containing glycyrrhizinic acid).

Some people only use Conn's syndrome for when it occurs due to an adrenal adenoma (a type of benign tumor). In practice, however, the terms are often used interchangeably, regardless of the underlying physiology.

The definitive diagnosis of HN requires morphological examination. Common histological features can be identified in the renal and glomerular vasculature. Glomerulosclerosis is often present, either focally or globally, which is characterized by hardening of the vessel walls. Also, luminal narrowing or the arteries and arterioles of the kidney system. However, this type of procedure is likely to be preceded with a provisional diagnosis based on laboratory investigations.

For people considered likely to have PAH based on the above tests, the specific associated condition is then determined based on the physical examination, medical/family history and further specific diagnostic tests (for example, serological tests to detect underlying connective tissue disease, HIV infection or hepatitis, ultrasonography to confirm the presence of portal hypertension, echocardiography/cardiac magnetic resonance imaging for congenital heart disease, laboratory tests for schistosomiasis, and high resolution CT for PVOD and pulmonary capillary hemangiomatosis). Routine lung biopsy is discouraged in patients with PAH, because of the risk to the patient and because the findings are unlikely to alter the diagnosis and treatment.

Although pulmonary arterial pressure (PAP) can be estimated on the basis of echocardiography, pressure measurements with a Swan-Ganz catheter inserted through the right side of the heart provide the most definite assessment.[42] Pulmonary hypertension is defined as a mean PAP of at least 25 mm Hg (3300 Pa) at rest, and PAH is defined as precapillary pulmonary hypertension (i.e. mean PAP ≥ 25 mm Hg with pulmonary arterial occlusion pressure [PAOP] ≤ 15 mm Hg and pulmonary vascular resistance [PVR] > 3 Wood Units). PAOP and PVR cannot be measured directly with echocardiography. Therefore, diagnosis of PAH requires right-sided cardiac catheterization. A Swan-Ganz catheter can also measure the cardiac output; this can be used to calculate the cardiac index, which is far more important in measuring disease severity than the pulmonary arterial pressure.

"Mean" PAP (mPAP) should not be confused with systolic PAP (sPAP), which is often reported on echocardiogram reports. A systolic pressure of 40 mm Hg typically implies a mean pressure of more than 25 mm Hg. Roughly, mPAP = 0.61•sPAP + 2.

Selective shunts select non-intestinal flow to be shunted to the systemic venous drainage while leaving the intestinal venous drainage to continue to pass through the liver. The most well known of this type is the splenorenal. This connects the splenic vein to the left renal vein thus reducing portal system pressure while minimizing any encephalopathy. In an H-shunt, which could be mesocaval (from the superior mesenteric vein to the inferior vena cava) or could be, portocaval (from the portal vein to the inferior vena cava) a graft, either synthetic or the preferred vein harvested from somewhere else on the patient's body, is connected between the superior mesenteric vein and the inferior vena cava. The size of this shunt will determine how selective it is.

It should be noted that with the advent of transjugular intrahepatic portosystemic shunting (TIPS), portosystemic shunts are less performed. TIPS has the advantage of being easier to perform and doesn't disrupt the liver's vascularity.

The diagnosis of portopulmonary hypertension is based on hemodynamic criteria:

1. . Portal hypertension and/or liver disease (clinical diagnosis—ascites/varices/splenomegaly)

2. . Mean pulmonary artery pressure—MPAP > 25 mmHg at rest

3. . Pulmonary vascular resistance—PVR > 240 dynes s cm−5

4. . Pulmonary artery occlusion pressure— PAOP 12 mmHg where TPG = MPAP − PAOP.

The diagnosis is usually first suggested by a transthoracic echocardiogram, part of the standard pre-transplantation work-up. Echocardiogram estimated pulmonary artery systolic pressures of 40 to 50 mm Hg are used as a screening cutoff for PPH diagnosis, with a sensitivity of 100% and a specificity as high as 96%. The negative predictive value of this method is 100% but the positive predictive value is 60%. Thereafter, these patients are referred for pulmonary artery catheterization.

The limitations of echocardiography are related to the derivative nature of non-invasive PAP estimation. The measurement of PAP by echocardiogram is made using a simplified Bernoulli equation. High cardiac index and pulmonary capillary wedge pressures, however, may lead to false positives by this standard. By one institution’s evaluation, the correlation between estimated systolic PAP and directly measured PAP was poor, 0.49. For these reasons, right heart catheterization is needed to confirm the diagnosis.

Secondary refers to an abnormality that indirectly results in pathology through a predictable physiologic pathway, i.e., a renin-producing tumor leads to increased aldosterone, as the body's aldosterone production is normally regulated by renin levels.

One cause is a juxtaglomerular cell tumor. Another is renal artery stenosis, in which the reduced blood supply across the juxtaglomerular apparatus stimulates the production of renin. Likewise, fibromuscular dysplasia may cause stenosis of the renal artery, and therefore secondary hyperaldosteronism. Other causes can come from the tubules: Hyporeabsorption of sodium (as seen in Bartter and Gitelman syndromes) will lead to hypovolemia/hypotension, which will activate the RAAS.