Potassium of 3.5 mM: Would you Treat?

It depends.

This is considered to be a 'normal' lab value.

CLINICAL EXAMPLE :
For young, healthy Patients that are asymptomatic, it may well be that nothing needs to be done.
*  Rx: Reassurance.

CLINICAL EXAMPLE:
For Patients with cardiac issues, it may be that the issue may resolve with increasing dietary intake of potassium.
*  Rx: Increase per os (po) dietary consumption.^

CLINICAL EXAMPLE:
For an elderly lady on chemotherapy as an inpatient who is asymptomatic, take a look at the History. You see that she has a history of cardiac issues.  
*  Rx: po potassium may be best. But watch for Tumor Lysis Syndrome, which kills tumor cells, releasing K into the serum. Elevated K can reach emergency levels.

CLINICAL EXAMPLE:
Yet another clinical example is a male patient in the Cardiac Care Care Unit (CCU) who is recovering from a myocardial infarction.  He has no complaints, his electrocardiogram is normal, and the K of 3.5 was a discovered on routine morning labs. The Patient is Postoperative Day (POD) #1, and still intubated on a ventilator.
*  Rx: Many Cardiac Surgeons prefer the K to be over 4.0 mM, for cardiac stability.
Remember to note the ventilator settings and any recent arterial blood gas (ABG) value. If you note hyperventilation, especially with an alkaline pH, remember that hyperventilation drives K into the cells. Take this into consideration, as the serum K will increase as soon as hyperventilaion is discontinued.
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Whoa. For astute Patients or Housestaff, let us just back up a moment. 

What are the presenting Symptoms of hypokalemia?

What are the "normal" values for potassium?

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LABORATORY VALUES FOR SERUM POTASSIUM:

*  K > 3.0 mM may be 'normal' and nonemergent. A 'relative' hypokalemia may exist if the Patient's   K is usually above 4.0, especially for Patients with cardiac issues.

*  K between 2.5 mM and 3.0 mM =          Moderate hypokalemia

*  K less than 2.5 mM                     =          Severe hypokalemia
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SYMPTOMS OF HYPOKALEMIA:

*  Mild Hypokalemia: usually has no symptoms.
*  Moderate Hypokalemia: Disorientation, confusion, muscle discomfort, muscle aches, and weakness.
*  Severe Hypokalemia: severe muscle weakness, flaccid paralysis, arrhythmia, bradycardia, and/or cardiac arrest.


 ^ Dietary Consumption helps; fresh fruits and vegetables are encouraged:

Fruit:             bananas, avocados, oranges, cantaloupe, strawberries, apricots, kiwi
Vegetables:    green roughage, tomatoes, peas, mushrooms, beets
Juices:           orange, grapefruit, apricot, prune
Other:           potatoes, beef, fish, turkey


 REFERENCES:
(1)  http://health.yahoo.net/channel/low-blood-potassium_symptoms
(2) http://emedicine.medscape.com/article/767448-treatment
(3) http://www.medicinenet.com/low_potassium_hypokalemia/article.htm