Hyperkalemia diagnostic study of choice
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:
Overview
Diagnostic Study of Choice
Study of choice
[Serum potassium] is the gold standard test for the diagnosis of [hyperkalemia].
There are two methods to determine serum potassium :
- Flame emission spectrophotometry
- Ion-specific electrode (ISE) potentiometry
ISE potentiometry has two different subtypes: Direct (undiluted) and indirect (diluted).
Direct ISE measures plasma potassium directly from a whole-blood sample and it's not associated with either pseudohyperkalemia.
FES or indirect ISE requires sample dilution before assay and both are associated with pseudohyperkalemia.
Pseudohyperkalemia
Pseudohyperkalemia is defined when serum potassium concentration exceeds that of plasma without any symptoms of hyperkalemia.It usually occurs when potassium moves out of cells during blood specimen collection or during centrifugation of the sample.Other causes are thrombocytosis,leukocytosis and erythrocytosis.To rule out pseudohyperkalemia we need to do the following
*Repeat the blood sample
*Complete blood count to rule out thrombocytosis,erythrocytosis and leukocytosis
*Measurement of plasma potassium and whole blood potassium
Diagnostic results
The finding on performing the diagnostic test that confirms hyperkalemia
*Serum Potassium level more than 5.1 meq/L.
Sequence of Diagnostic Studies
The [serum potassium] must be performed when:
- The patient presented with cardiac arrhythmias,weakness,fatigue and known case of chronic kidney disease as the first step of diagnosis.
The various investigations must be performed :
- Blood pressure(to look for hypoaldosteronism)
- Complete blood count
- Renal function tests
- Urine potassium,sodium and osmolality
- Metabolic profile(other electrolytes)
- ECG
- Bicarbonate level
- Serum glucose
- Serum Calcium
Depending on the history and results of the above mentioned tests,other tests that can be performed for evaluating the cause of hyperkalemia.
- Digoxin level - If the patient is on a digitalis medication
- Arterial or venous blood gas
- Urinalysis
- Serum cortisol and aldosterone levels
- Serum uric acid and phosphorus assays
- Serum creatinine phosphokinase (CPK) measurements
- Urine myoglobin test
Sequence
- Serum Potassium measurement
- ECG-It denotes the urgency of the treatment
- Renal function test
Serum BUN and creatinine are measured.
Since creatinine levels are dependent on muscle mass so GFR measurement is preferred
- Urine potassium,sodium and osmolality measurement
Urine potassium measurement
Urine potassium <20meq/L denotes impaired excretion of potassium and denotes renal cause of hyperkalemia.
Urine potassium .40meq/L denotes adequate excretion of potassium and excludes renal cause of hyperkalemia.
Urine sodium <20meq/L denotes decreased sodium delivery to the distal tubules which decreases potassium secretion.
Urine osmolarity
measuring urine osmolarity is very important for accurate measurement of urine potassium as concenterated or dilute urine will alter the urine potassium concenteration.
- Serum osmolarity
High serum osmolarity(>295 mosm/kg) may result in extracellular shift of potassium .
- Blood gas analysis
Decreased serum ph causes extracellular shift of potassium into the blood
- Transtubular Potassium gradient
It calculates the ratio of amount potassium in the collecting duct of kidneys with the amount of potassium in the peritubular capillaries.
It indicates the activity of aldosterone on kidneys in regulation of potassium levels.
TTG calculation-( Urine K+ X Serum osmolarity)/(serum K+ X Urine omolarity)
TTG <3 suggests lack of aldosterone effect on collecting ducts causing decreased excretion of potassium.
TTG >7 suggest adequate effect of aldosterone in a case of hyperkalemia.
If TTG suggest aldosterone etiology then further testing done
Aldosterone levels
Renin levels