The Potassium Draw: Special Measures

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When evaluating patients for periodic paralysis it is vital to 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.

From "A Manual of Laboratory Diagnostic Tests" by Frances Fischbach, Page 247-253.

Potassium (K+) Normal Values: 3.5-5.0 mmol/liter 4.0-4.7 mmol/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; as hemolysis liberates K+ from red blood cells and causes falsely elevated results. Sample must be delivered to laboratory and examined at once to separate cells from serum. Potassium leaks out of the cell and sample will be hemolyzed after four hours.

Interfering Factors

Forearm exercise; Opening and closing the fist 10 times with a tourniquet in place results in a 10-20% increase of the serum K+ level. 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the tourniquet be released after the needle has entered the vein and two minutes allowed to elapse before the sample is withdrawn.

 

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