Editorial: Sample is key for a Correct Aldosterone-Renin Ratio (ALRR)
Most laboratory tests require that one specific sample be obtained. However, some tests use a comparison of two different analytes, often as a ratio. When these analytes require different types of samples, delays in testing or erroneous results can occur if proper attention is not given to the initial sample itself.
ALRR is an effective screening test for patients with Conn’s syndrome, primary hyperaldosteronism. This uncommon cause of hypertension (about 1% of hypertensive patients) is usually the result of a spontaneous hyperplasia (bilateral or unilateral) or a tumor (usually benign) of the adrenal cortex that produces excessive amounts of aldosterone. Normally, aldosterone functions to conserve sodium by enhancing its reabsorption by exchanging it for potassium in the kidney tubules. When there is excessive production of aldosterone, the level of sodium increases with resulting water retention and a decrease in potassium levels. This imbalance leads to the unusual combination of hypokalemia and hypertension in about 20-30% of these patients, a combination that should cause one to examine the level of aldosterone. It is less appreciated, however, that most patients with Conn’s syndrome will have serum potassium in the normal range. Because of this, The Clinical Guidelines of the Endocrine Society recommend an ALRR screen in higher risk groups of hypertensive patients and those with hypokalemia. Renin is produced by the juxtaglomerular apparatus in the kidney and the increased blood volume and blood pressure inhibits the release of renin.
However, an increase in aldosterone is not diagnostic for Conn’s syndrome because aldosterone can increase from other causes. When there is hypoperfusion of the kidneys due to vascular problems such as renal artery stenosis (a condition more common than Conn’s syndrome) the level of renin responds with an increase to stimulate aldosterone production thereby helping to restore the blood pressure. This condition is termed secondary aldosteronism. Because both conditions have an increased aldosterone the difference in renin levels is a key to the correct diagnosis and use of the ALRR has been found to enhance the specificity and sensitivity of the test.
Unfortunately, each test needs a different sample and that can results in a delay if either sample is not drawn in the proper tube and/or maintained at the proper temperature.
When an ALRR is ordered, the labels for the tubes do not say ALDOS or PRA. The labels for both tubes are ALRR.
Tube 1. For aldosterone we need at least 1.0 ml of serum drawn in an SST Tube. It must be separated and refrigerated (4°C), or it may be frozen. If the sample is ordered through our computer, the label “ALRR 5.0 ml SST/Corv tube Refrigerate” will be printed out. Please affix this to the tube. A plasma sample cannot be used.
Tube 2. The plasma renin activity (PRA) requires a lavender top tube (EDTA plasma). Centrifuge the cells, separate and freeze the plasma immediately. It is CRITICAL for this sample to be frozen. A refrigerated sample cannot be used. If the sample is ordered through our computer, affix the label that reads “ALRR 7.0 ml LAV Top Sep/FRZ IMMED” to the frozen specimen.
If we do not get acceptable samples for both portions of the ALRR we cannot report any results.
Please feel free to call Client Services at Warde Medical Laboratory (800 760-9969) to clarify any questions about this testing.
Reference: J Clin Endo Metab 93:3266-3281, 2008.