Chemical Pathology JC132: Biochemical Investigation Of Hypertension Flashcards
Biochemical investigation for HT
Aim:
1. Locate a secondary causes of HT (if any)
Endocrine causes:
- ***Mineralocorticoid excess syndromes (MES)
—> Primary aldosteronism
—> Cushing syndrome
- Hyperthyroidism
- Phaeochromocytoma (Tumour of adrenal glands + ganglions)
- Renal parenchymal diseases
Structural causes:
- OSA
Vascular causes:
- Coarctation of aorta
- Prevent + screen for complications
- Renal complications, Renal insufficiency, Proteinuria
- Cardiac complications
- Cerebrovascular complications - Detect + treat co-morbidities
- DM: FG, HbA1c
- Dyslipidaemia: Lipid profile
Mineralocorticoid excess
Mineralocorticoid (Steroids that activate mineralocorticoid receptors):
- ***Aldosterone
- ***Cortisol
- ***Deoxycorticosterone
Effects of Mineralocorticoid excess:
- ***Hypokalaemic hypertension (K excretion, Na retention)
- ***Metabolic alkalosis
- LVH (∵ HT)
Causes of Mineralocorticoid excess
- Mineralocorticoid excessive
- Aldosterone: **Primary aldosteronism: adenoma vs hyperplasia, familial vs sporadic, **Secondary aldosteronism
- Cortisol: **Ectopic ACTH syndrome, **Glucocorticoid resistance
- ***Deoxycorticosterone (11-hydroxylase deficiency, 17-hydroxylase deficiency) (also cause HT) - Mineralocorticoid receptor
- Activating mutation
- Apparent mineralocorticoid excess, Liquorice - Signaling protein
- Gordon syndrome (Hyperkalaemia, Hypertension) - Sodium channel
- Liddle syndrome
Glucocorticoid remediable aldosteronism
Biochemical picture:
- Same as Primary aldosteronism (except run in family, AD manner)
Fusion of 2 genes:
- 5’ of CYP11B1 (11β hydroxylase, regulated by ACTH)
- 3’ of CYP11B2 (Aldosterone synthase, regulated by angiotrensin 2)
—> Result: Activation of aldosterone synthesis by ***ACTH (which should not be the case, usually aldosterone mainly produced via RAAS system)
Biochemical testing:
- ***Dexamethasone suppression (∵ promoter can be suppressed by Dexamethasone)
- Measurement of ***hybrid steroids 18-hydroxycortisol + 18-oxocortisol
- Genetic testing: Long-range PCR (fusion of Promoter of 11β hydroxylase gene + Aldosterone synthase gene)
How to measure mineralocorticoid activity?
- Measurement of individual mineralocorticoid
- **Blood Aldosterone, Urine metabolites (tetrahydroaldosterone etc.)
- **Cortisol, Cortisone, Urinary metabolites (normally do not have mineralocorticoid activity, only when in excess)
- Deoxycorticosterone -
Transtubular K gradient (TTKG)
- normally 4-10
- reflects mineralocorticoid activity (K excretion)
- (K urine / Osm urine) ÷ (K serum / Osm serum)
- prerequisite (otherwise not interpretable):
—> Urine osm > Serum osm
—> Urine Na >40
—> No K supplementation
Primary aldosteronism
- High prevalence: 8-12% in most countries
Group of disorders:
- Aldosterone production inappropriately high for Na level
- Relatively ***autonomous of major regulators of secretion (Angiotensin 2, Plasma K)
- ***Non-suppressible by Na loading
**Classical presentation:
1. High aldosterone
2. Undetectable renin (∵ negative feedback)
—> **↑ Aldosterone-renin ratio (ARR)
3. ***HypoK
Patients group with highest risk of PA:
- Severe HT, Drug-resistant
- Spontaneous / Diuretic-related HT
- Early onset HT / CVA at young age
- Relatives of patients with PA
What test to use:
- Screening: ***Aldosterone-renin ratio (ARR)
- Confirmation: >=1 of 4 confirmatory tests
- Oral sodium loading test
- ***Saline infusion test
- Fludrocortisone suppression test
- Captopril challenge test - Confirmation not necessary in patients with classical presentation of **High aldosterone, **Undetectable renin, ***HypoK —> too typical of PA
Subtyping:
- CT to exclude to cancers + assist radiologists / surgeons
***Aldosterone-renin ratio (ARR)
Can have False positive / False negative
-
**Drugs
- Most drugs have more significant effect on **Renin than Aldosterone —> effect depends on drug action on Renin level rather Aldosterone
- NB: Renin inhibitors act differently depending on whether mass / activity assay used - **K level in body (mediated through **Aldosterone)
- **Salt diet (mediated through **Renin)
- Old age
- Pre-menopausal women
- Pregnancy
- Renal artery stenosis
Examples:
- β blockers, Central agonist (Clonidine, Methyldopa), NSAIDs
—> ***↓ Renin, ↓ Aldosterone
—> Renin ↓ more —> ↑ ARR —> False +ve
- All diuretics (K wasting / K sparing)
—> ↑ Aldosterone + ***↑ Renin (Renin ↑ more)—> ↓ ARR —> False -ve - ACEI, ARB
—> ↓ Aldosterone + ***↑ Renin —> ↓ ARR —> False -ve
Potassium effect on ARR
- K: Significant regulator of Aldosterone secretion
- Effects generally ***not mediated by Renin
Therefore:
- Must correct K prior to testing —> otherwise difficult to interpret results
- HypoK —> Low Aldosterone —> ↓ ARR —> False -ve
- HyperK —> High Aldosterone —> ↑ ARR —> False +ve
Na effect on ARR
Na loading inhibits Aldosterone secretion via ***Renin
Therefore:
- Normal salt diet before investigations
- High salt —> ↓↓ Renin + ↓ Aldosterone secretion —> ↑ ARR —> False +ve
- Low salt —> ↑↑ Renin + ↑ Aldosterone secretion —> ↓ ARR —> False -ve
Old age, Pre-menopausal women, Pregnancy, Renal artery stenosis effect on ARR
Old age
- ↓↓ Renin + ↓ Aldosterone secretion —> ↑ ARR —> False +ve
- **Pre-menopausal women
- ↓ Renin + ↑ Aldosterone secretion —> ↑ ARR —> False +ve
Pregnancy
- ↑↑ Renin + ↑ Aldosterone secretion —> ↓ ARR —> False -ve
Renal artery stenosis (Secondary aldosteronism)
- ↑↑ Renin + ↑ Aldosterone secretion —> ↓ ARR —> False -ve
How to measure ARR
- Out of bed for >=2 hours
- Seated for 5-15 mins
- Unrestricted salt intake (Supra) (some centre salt-load)
- Drugs: not to withhold drug but interpret results with reference to drug regimen
—> BUT **Mineralocorticoid receptor antagonist (e.g. Spironolactone, Eplerenone), **Diuretics, **Liquorice must be withdrawn for **>=4 weeks prior to testing
Best Anti-HT prior to ARR testing:
- ***Verapamil SR (Non-Dihydropyridine CCB)
- ***Hydralazine
- ***α blockers (Prazosin, Doxazocin, Terazocin)
Renin:
- no centre in HK utilise direct renin concentration (measure activity rather than concentration)
- currently all assays used in HK are ***LC-MS/MS (LC + tandem MS)
Aldosterone:
- measured by LC-MS/MS
Cut-off for ARR
Differs from centre to centre
- guideline: 750 (renin: ng/ml/hr, aldosterone: pmol/L)
- different hospitals in HK: 400-1000
No absolute gold standard of values, depends on local practice
4 Confirmatory tests
- Oral sodium loading test
- give Na to suppress Aldosterone —> non-suppressible
- measure urine Aldosterone - Saline infusion test
- give Na to suppress Aldosterone —> non-suppressible
- measure plasma Aldosterone - Fludrocortisone suppression test
- considered by some as gold standard, exogenous mineralocorticoid —> non-suppressible
- measure plasma Aldosterone (>6 on day 4 at 10am confirms PA) - Captopril challenge test
- aldosterone non-suppressible by captopril, renin activity remain suppressed
- considered by some as not-so-good
Familial Primary Aldosteronism
3 types:
- FH-1: Glucocorticoid remediable aldosteronism
- FH-2: AD (gene not identified, molecular basis unclear)
- FH-3: Germline mutation in KCNJ5 (Kir3.4: mutation causes ↑ Na conductance + depolarisation —> ↑ Aldosterone secretion)
Balance test
Patient prepared same as ARR study
- Supine sample: fast after midnight, supine till 9am, obtain blood for renin + aldosterone
- Erect sample: ambulate for 4 hours
- Bilateral adrenal hyperplasia: Exaggerated physiological response of ↑ of Aldosterone at upright position (∵ ↑ renin activity)
- Adenoma: Paradoxical ↓ of Aldosterone (probably ∵ ACTH ↓ in diurnal rhythm, accuracy ~85% ∵ some aldosterone-secreting tumours still responsive to Angiotensin 2)
