The Potassium Draw: Special Measures
When evaluating patients for periodic paralysis it is vital that you obtain accurate results from blood draws. Because potassium levels which vary only a fraction of a point may be enough to trigger weakness in some patients, the lab may need to be made aware of specialized procedures which should ideally be used to obtain samples for K+ evaluation.
Potassium (K+)
Normal Values
3.5-5 mEq/liter
4-4.7 mEq/liter (average)
Potassium is the principal electrolyte (cation) of intracellular fluid, and primary buffer within the cell itself. Ninety percent of K+ is concentrated within the cell; only small amounts are contained in the bone and blood. A kilogram of tissue such as RBC or muscle contains about 90 mEq. of K+. Damaged cells release K+ into the blood.
Potassium plays an important role in nerve conduction and muscle function. Moreover, it helps maintain acid-base balance and osmotic pressure. Along with calcium and magnesium, potassium controls the rate and force of contraction of the heart and thus, cardiac output. Evidence of a potassium deficit can be noted on an electrocardiogram (ECG) by the presence of a U wave. With mild hypokalemia electrocardiogram (ECG) may show S-T depression, flattened T waves, presence of U waves With severe hypokalemia ECG may show peaked P waves, prolonged P-R interval, widened QRS.
Procedure
- A venous blood sample of 10 ml is obtained.
- Hemolysis in obtaining the sample should be avoided; it will give false-to-high results.
- Sample must be delivered to laboratory and examined at once to separate cells from serum. Potassium leaks out of the cell and will be falsely high after four hours.
Interfering Factors
Forearm exercise
Opening and closing the fist 10 times with a tourniquet in place results in an increase of the K+ level by 10% to 20%. For this reason it is recommended that the blood sample be obtained without a tourniquet, or that the tourniquet be released after the needle has entered the vein and two minutes allowed to elapse before the sample is withdrawn.
From "A Manual of Laboratory Diagnostic Tests" by Frances Fischbach, Page 247-253.
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