Blood Tests for High Blood Pressure
The cuff measures the number. The blood and urine tests that follow do three separate jobs: check for kidney damage, look for a treatable underlying cause, and quantify your overall cardiovascular risk.
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The Quick Answer
Blood pressure itself is measured with a cuff, not a blood test. Once a reading is confirmed as genuinely high, your GP orders a set of blood and urine tests that do three separate jobs, none of which the cuff reading can tell you on its own.
Job one is to look for end-organ damage, meaning whether the pressure has already started to affect the kidneys. Job two is to look for a treatable secondary cause, such as an aldosterone-producing adrenal gland, that a tablet alone will not fix. Job three is to quantify your overall cardiovascular risk, because the blood pressure number matters most in the context of your cholesterol, glucose, and other risk factors combined.
Most people have no single identifiable cause for their high blood pressure. That is normal, and it is called essential or primary hypertension. The tests below exist to catch the meaningful minority who do have a specific, sometimes curable, driver.
Typical Australian Reference Ranges
Ranges vary slightly between Australian laboratories. Always read your own report against the range printed on it, not this general guide.
Sodium
135-145 mmol/L
Low sodium with high blood pressure can occur with some diuretics or, rarely, adrenal disease
Potassium
3.5-5.2 mmol/L
The single most useful screening clue: persistently low-normal or low potassium raises suspicion of primary aldosteronism
Creatinine
60-110 micromol/L (men), 45-90 micromol/L (women)
Rises as kidney function falls; interpreted alongside eGFR, age, and muscle mass
eGFR
Above 90 mL/min/1.73m² (age-adjusted)
Below 60 for three months or more defines chronic kidney disease
Urine ACR (UACR)
Below 2.5 mg/mmol (men), below 3.5 mg/mmol (women)
The earliest marker of hypertensive kidney damage, well before creatinine changes
Fasting total cholesterol
Below 5.5 mmol/L (general target)
Interpreted as part of the overall lipid profile and absolute cardiovascular risk, not in isolation
HbA1c
Below 42 mmol/mol (6.0%)
42-47 mmol/mol is pre-diabetes; 48 mmol/mol (6.5%) or above is diagnostic of diabetes
TSH
Approximately 0.4-4.0 mIU/L
Both an overactive and an underactive thyroid can raise blood pressure
Corrected calcium
2.10-2.60 mmol/L
Persistently high calcium raises suspicion of primary hyperparathyroidism
Job One: Checking for End-Organ Damage
High blood pressure damages small blood vessels slowly and without symptoms. The kidneys, with their dense network of tiny filtering vessels, are usually the first organ to show measurable change, well before symptoms appear.
Creatinine and eGFR
Creatinine is a waste product cleared by the kidneys; a rising level suggests falling function. eGFR (estimated glomerular filtration rate) converts creatinine, age, and sex into an estimate of how well the kidneys are filtering, expressed as a percentage of normal. An eGFR below 60 mL/min/1.73m² on more than one occasion, three months apart, meets the definition of chronic kidney disease. In hypertension, creatinine and eGFR serve two purposes: they establish a baseline before treatment starts, and they get rechecked afterwards to make sure the kidneys are tolerating the chosen medication (see the monitoring table further down this page).
Urine albumin to creatinine ratio (UACR): the test most often left off
UACR is a spot urine test that detects tiny amounts of albumin leaking into the urine, a phenomenon called microalbuminuria. It is the single earliest detectable sign that high blood pressure is damaging the kidney filters, appearing well before creatinine rises or eGFR falls. It also independently predicts cardiovascular risk, on top of the blood pressure reading itself. Despite costing very little and using a simple urine sample rather than a blood draw, it is the test most commonly left off a hypertension work-up in busy general practice. If your GP has only ordered blood tests and not a urine ACR, it is reasonable to ask whether one should be added.
Full blood count (FBC)
A full blood count is included partly as a general baseline before starting any new medication, and partly because a low haemoglobin can contribute to a false impression of how the heart is coping, while a raised haemoglobin or haematocrit occasionally points towards an underlying secondary cause or towards untreated obstructive sleep apnoea. It is a supporting test in this context rather than a central one.
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Job Two: Looking for a Treatable Secondary Cause
Roughly 10 per cent of adults with hypertension, and a considerably higher proportion of those whose blood pressure is difficult to control, have an identifiable underlying driver. Electrolytes are the first clue, and specific further tests follow when the picture fits.
Potassium and sodium: the first clue
A low or low-normal potassium in someone with high blood pressure is the classic textbook clue to primary aldosteronism. In that condition one or both adrenal glands overproduce the hormone aldosterone, which makes the kidney retain sodium and lose potassium. Primary aldosteronism is now understood to be far more common than was once taught, and most people who have it have a normal potassium result, so a normal level does not rule it out on its own. It is one of the few genuinely curable causes of hypertension: a single adenoma can sometimes be removed surgically, and bilateral disease responds well to a specific class of medication (a mineralocorticoid receptor antagonist, such as spironolactone).
The aldosterone to renin ratio (ARR), and why medications matter first
When primary aldosteronism is suspected, the screening test is the aldosterone to renin ratio. This is where medications become important: spironolactone, ACE inhibitors, ARBs, beta-blockers, and dihydropyridine calcium channel blockers can all distort renin levels enough to produce a false result. Where practical, the test is timed for when the patient is on medications that interfere least (such as verapamil slow-release, hydralazine, or prazosin), or spironolactone is withheld for roughly six weeks beforehand. In practice, many GPs test on current medications first as a pragmatic screen and interpret a borderline result cautiously, referring for formal confirmation testing (such as a saline infusion test) if the ratio is suggestive.
Primary aldosteronism
Clue: Low or low-normal potassium; blood pressure resistant to three or more medications; onset before age 40; a family history of early hypertension or stroke
First test: Aldosterone to renin ratio (ARR), ideally with interfering medications adjusted first
Prevalence estimates vary widely with the population studied and the screening method used, and are commonly cited at around 5 to 10 per cent of all people with hypertension, higher again in resistant hypertension. It is far more common than once believed, and it is one of the few genuinely curable causes.
Thyroid disease (over- or underactive)
Clue: Weight change, heat or cold intolerance, palpitations, tremor, fatigue, change in bowel habit
First test: TSH, with free T4 if TSH is abnormal
Both hyperthyroidism and hypothyroidism can raise blood pressure by different mechanisms, and both are straightforward to treat once identified.
Primary hyperparathyroidism
Clue: Persistently high corrected calcium, kidney stones, bone pain, constipation, low mood; often found incidentally
First test: Corrected calcium, followed by parathyroid hormone (PTH) if calcium is raised
Uncommon as a cause of hypertension specifically, but calcium is worth checking once, since it costs very little to add to the same blood draw.
Phaeochromocytoma or paraganglioma
Clue: Episodic pounding headache, sweating, and palpitations; wildly variable or paroxysmal blood pressure; young age of onset; relevant family history
First test: Plasma free metanephrines, or 24-hour urine metanephrines and catecholamines
Rare, and tested for only when the story specifically fits rather than in every new diagnosis of hypertension.
Cushing's syndrome (excess cortisol)
Clue: Central weight gain, thin skin that bruises easily, purple stretch marks, muscle weakness, round face, high glucose alongside high blood pressure
First test: Overnight dexamethasone suppression test or late-night salivary cortisol, arranged if suspected
Rare, but corticosteroid medication use should always be asked about first, as it is a far more common cause of the same picture.
Medication and substance-induced
Clue: Regular NSAIDs, oral decongestants (pseudoephedrine), the combined oral contraceptive pill, liquorice in large quantities, corticosteroids, some antidepressants, excess alcohol
First test: No blood test; identified by taking a careful medication and substance history
This is one of the most common and most easily reversed contributors, and it is worth reviewing before pursuing rarer causes.
Job Three: Quantifying Cardiovascular Risk
High blood pressure rarely occurs in isolation. It contributes to your overall risk of a heart attack or stroke alongside your cholesterol, blood glucose, smoking status, age, and sex, and Australian guidelines treat that combined risk, not the blood pressure reading alone, as the main driver of how urgently to act.
Fasting lipids, non-HDL cholesterol, and ApoB
A standard fasting lipid profile (total cholesterol, HDL, LDL, and triglycerides) is ordered alongside the hypertension work-up. Non-HDL cholesterol (total cholesterol minus HDL) captures all the atherogenic particles in one number and does not require fasting to be accurate, which makes it a practical addition. ApoB, a direct count of atherogenic particles, is increasingly used when the standard lipid profile is discordant or when risk assessment is borderline, though it is not yet part of every standard panel and is sometimes a private-pay addition.
HbA1c or fasting glucose
Diabetes and pre-diabetes both substantially raise cardiovascular risk on top of high blood pressure, and the combination is common enough that HbA1c or fasting glucose is a routine part of the same blood draw rather than a separate investigation. An HbA1c of 48 mmol/mol (6.5%) or above, or a fasting glucose of 7.0 mmol/L or above, is diagnostic of diabetes and materially changes both the blood pressure target and the choice of medication.
The Australian absolute cardiovascular disease risk calculator
Once the lipids, glucose, and blood pressure results are in, your GP enters them into the Australian absolute cardiovascular disease (CVD) risk calculator, along with your age, sex, and smoking status. The output is a single percentage: your estimated chance of a heart attack or stroke in the next five years. A high percentage supports starting blood pressure and, often, cholesterol-lowering medication earlier and more assertively, even at blood pressure levels that might otherwise be managed with lifestyle measures alone.
Baseline and Follow-Up Blood Tests After Starting Medication
Starting or changing blood pressure medication is not a one-off event. Each class has its own expected blood test changes and its own follow-up schedule.
ACE inhibitor (e.g. perindopril, ramipril) or ARB (e.g. irbesartan, telmisartan)
Baseline: Electrolytes, creatinine, and eGFR before starting
Follow-up: Repeat electrolytes and creatinine at 1-2 weeks after starting or any dose increase
Watch for: A creatinine rise of up to 20-30% from baseline is expected and reflects the intended drop in pressure inside the kidney filter. A larger rise, or potassium above 5.5-6.0 mmol/L, needs review and sometimes a dose reduction.
Thiazide or thiazide-like diuretic (e.g. indapamide, hydrochlorothiazide)
Baseline: Electrolytes, creatinine, fasting glucose
Follow-up: Repeat electrolytes at 2-4 weeks after starting, then periodically
Watch for: Low sodium and low potassium are the main risks, along with a small rise in glucose and urate. More pronounced in older patients and at higher doses.
Spironolactone (often added for resistant hypertension or confirmed primary aldosteronism)
Baseline: Electrolytes and creatinine before starting; confirm potassium is not already elevated
Follow-up: Repeat potassium at 1 week, then at 1 month, then periodically, more frequently if also on an ACE inhibitor or ARB
Watch for: High potassium is the main risk, especially in older patients, those with reduced kidney function, or those also taking an ACE inhibitor or ARB. Breast tenderness is a common non-laboratory side effect.
When to Suspect Obstructive Sleep Apnoea
Obstructive sleep apnoea (OSA) does not show up on a blood test, but it is one of the most common and most frequently missed contributors to high blood pressure, so it is worth covering here alongside the tests that do get ordered.
Suspicion should rise with loud snoring, witnessed pauses in breathing during sleep (usually reported by a partner), waking unrefreshed despite adequate hours in bed, morning headaches, and marked daytime sleepiness. A larger neck circumference and excess weight carried around the abdomen both increase the likelihood. Blood pressure that stays high overnight rather than dipping as it normally should, or that resists three or more medications at good doses, is a particularly strong clue.
There is no blood marker for OSA. Clinical suspicion based on the features above leads to a home or in-laboratory sleep study, and treating confirmed OSA, usually with continuous positive airway pressure (CPAP), can meaningfully lower blood pressure on top of whatever medication is already in place.
What Your GP Will Do Next
Confirm the diagnosis properly
A single elevated reading in the clinic is not enough to diagnose hypertension. Your GP will usually arrange home blood pressure monitoring or a 24-hour ambulatory blood pressure monitor to rule out "white coat" effects and confirm the average pressure over time, before or alongside the blood tests.
Order the standard hypertension work-up
Electrolytes and creatinine with eGFR, full blood count, fasting lipids, fasting glucose or HbA1c, and a urine albumin to creatinine ratio, usually from the one visit and often billed under a single Medicare item.
Look specifically at potassium and the UACR
These two results carry the most weight for deciding whether to look further. A low potassium prompts consideration of the aldosterone to renin ratio. Any degree of albuminuria on the UACR confirms that the kidneys are already affected and often changes the choice and intensity of treatment.
Calculate your absolute cardiovascular risk
Your GP combines your blood pressure, lipids, age, sex, smoking status, and diabetes status in the Australian absolute cardiovascular disease risk calculator. This percentage, more than the blood pressure number alone, guides how urgently to treat and how strict the targets should be.
Decide whether a secondary cause needs chasing
Most people with high blood pressure have no single identifiable cause (this is called essential or primary hypertension) and move straight to lifestyle advice and medication. Features such as a low potassium, resistant hypertension, young age of onset, or a specific symptom pattern change this, and prompt the additional tests covered above.
Start treatment and set a monitoring schedule
If medication is started, your GP will schedule the follow-up blood tests appropriate to that medication (see the monitoring table above) and a repeat blood pressure check, typically within 2-6 weeks, to confirm the target is being reached safely.
Refer where the picture warrants it
Suspected primary aldosteronism, renal artery stenosis, phaeochromocytoma, or blood pressure that remains uncontrolled on three or more medications at good doses (including a diuretic) usually warrants referral to a physician or hypertension specialist for further work-up.
When to Seek Urgent Care
Severe headache, visual disturbance, confusion, or chest pain with a very high reading
A blood pressure reading above roughly 180/120 mmHg together with any of these symptoms is a hypertensive emergency and needs urgent same-day medical assessment, usually in an emergency department, not a routine blood test appointment.
Sudden weakness, slurred speech, or facial drooping
These are stroke symptoms. Call 000 immediately regardless of the blood pressure reading.
Potassium above 6.0 mmol/L on a blood test
Significantly high potassium can affect heart rhythm and needs same-day medical review, particularly if you are taking an ACE inhibitor, ARB, or spironolactone.
A large, unexplained rise in creatinine after starting or increasing medication
A rise well beyond the expected 20-30% after starting an ACE inhibitor or ARB should prompt a phone call to your GP rather than waiting for the next scheduled review.
Episodes of pounding headache, sweating, and palpitations with very high readings
This pattern, especially if it comes in distinct episodes, should be discussed with your GP promptly, as it can indicate phaeochromocytoma, which is rare but important to identify.
Sources and reference ranges
Adult reference ranges on this page follow the AACB and RCPA harmonised reference intervals unless stated otherwise; the range printed on your own report always takes precedence because laboratories differ.
Blood Tests for High Blood Pressure: Frequently Asked Questions
Why do I need blood tests if my blood pressure is already measured with a cuff?
The cuff only tells you the number. It cannot tell you whether high blood pressure has already affected your kidneys or heart, whether a treatable underlying condition is driving it, or how it fits into your overall cardiovascular risk. Blood and urine tests do those three separate jobs. They check for early kidney damage, look for hormonal or endocrine causes that a tablet alone will not fix, and quantify your absolute risk of a heart attack or stroke so treatment decisions are based on more than the reading itself.
What blood tests are done when high blood pressure is first found?
A standard Australian workup after a new diagnosis of hypertension includes electrolytes and creatinine with eGFR, a full blood count, fasting lipids, fasting glucose or HbA1c, and a urine albumin to creatinine ratio (UACR). Most of these are billed under the one Medicare item as a "hypertension work-up". Further tests such as the aldosterone to renin ratio, TSH, calcium, or plasma metanephrines are added only when the story, examination, or initial results suggest a secondary cause is plausible.
What does the urine albumin to creatinine ratio (UACR) test for in hypertension?
UACR detects tiny amounts of albumin leaking into the urine, long before creatinine rises or eGFR falls. In someone with high blood pressure, a raised UACR is one of the earliest signs that the pressure is damaging the small filtering vessels in the kidney. It also independently predicts cardiovascular risk, on top of blood pressure itself. It is inexpensive, uses a spot urine sample, and is one of the tests most often left off this workup.
Can a low potassium result mean my high blood pressure has a treatable cause?
Yes, in a meaningful minority of cases. A low or low-normal potassium alongside high blood pressure is the classic, though not universal, clue to primary aldosteronism, a condition where the adrenal glands produce too much aldosterone. It is now understood to affect a substantially larger proportion of people with hypertension than was once thought, particularly those with harder-to-control blood pressure, and it is potentially curable with surgery or well controlled with a specific medication. Most people with primary aldosteronism have a normal potassium, so a normal result does not rule it out.
What is the aldosterone to renin ratio and why do medications interfere with it?
The aldosterone to renin ratio (ARR) is the screening test for primary aldosteronism. It compares the level of aldosterone to the level of renin, the hormone that normally drives aldosterone production. Many blood pressure medications, including spironolactone, ACE inhibitors, ARBs, and beta-blockers, change renin levels and can produce a false result in either direction. Wherever practical, the test is done on medications that interfere least, such as verapamil or prazosin, or spironolactone is stopped for around six weeks beforehand. Your GP will tell you which of your medications, if any, need to be adjusted before the sample is taken.
Do I need a cardiovascular risk calculation as well as blood tests?
Yes. High blood pressure is one input into your overall risk of a heart attack or stroke over the next five years, alongside your cholesterol, age, sex, smoking status, and diabetes status. The Australian absolute cardiovascular disease risk calculator combines all of these into a single percentage, and that percentage, not the blood pressure reading alone, is what usually determines how urgently treatment is started and how aggressive the targets should be. This is why fasting lipids and glucose are ordered alongside the kidney and hormone tests, even in someone who otherwise feels well.
What blood test changes should I expect after starting blood pressure medication?
After starting an ACE inhibitor or ARB, a small rise in creatinine (up to about 20 to 30 per cent from baseline) and eGFR is expected and reflects the intended reduction in pressure inside the kidney filters, not new damage. A larger rise needs review. Thiazide diuretics can lower potassium and sodium and occasionally raise glucose or urate. Spironolactone can raise potassium significantly, especially alongside an ACE inhibitor or ARB. Repeat electrolytes and creatinine are usually checked one to two weeks after starting or increasing any of these, then periodically once stable.
When should high blood pressure prompt testing for phaeochromocytoma?
Phaeochromocytoma is rare, but it is worth testing for with plasma or urine metanephrines when high blood pressure comes with episodes of pounding headache, sweating, and palpitations, when blood pressure is wildly variable or spikes dramatically, when it appears at a young age or is difficult to control on multiple medications, or when there is a family history of the condition or related genetic syndromes. Most people with everyday hypertension do not need this test; it is reserved for a story that specifically fits.
Could obstructive sleep apnoea be causing my high blood pressure?
Obstructive sleep apnoea (OSA) is one of the most common and most under-diagnosed secondary contributors to high blood pressure, especially blood pressure that stays high overnight or resists three or more medications. Loud snoring, witnessed pauses in breathing, waking unrefreshed, morning headaches, and daytime sleepiness are the usual clues, and OSA is more likely with a larger neck circumference or excess weight around the abdomen. There is no blood test for OSA; suspicion based on these features leads to a sleep study, and treating it can meaningfully lower blood pressure on top of medication.
Related Reading
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This page provides general educational information about the blood and urine tests commonly used to investigate high blood pressure. It is not a substitute for professional medical advice, diagnosis, or treatment. Always discuss your blood pressure readings and test results with your GP, who has access to your full medical history and can interpret them in context. SmarterBlood does not provide medical care.