Tuesday, January 28, 2014

Pseudohyperkalemia: Some questions and possible answers?

Pseudohyperkalemia- what’s the mechanism and why?

Classically, we observe this in patients who have thrombocytosis or leukocytosis. Potassium is released from WBCS and platelets when a blood sample is allowed to clot in vitro.   When we use a plasma sample, the tube it gets drawn is heparinized and hence clotting doesn’t occur and plasma K levels are usually lower than serum K levels. When you have thrombocytosis or leukocytosis, this process is even more prominent.   In thrombocytosis, platelet granules release K.

Besides the clotting leading to lysis and K release, prolonged storage of blood at room temperature or in cold before performing the test and lead to impair N/K ATPase pump in WBCS and lead to spurious K levels.

Some researchers have defined pseudohyperkalemia when serum potassium concentration exceeded that of plasma by more than 0.4 mmol/L provided that samples are collected under strict techniques, remain at room temperature and are tested within 1 hour from blood specimen collection.

The phenomenon of pseudohyperkalemia was first reported by Hartmann and Mellinkoff in 1955 as a marked elevation of serum potassium levels in the absence of clinical evidence of electrolyte imbalance. They also found that in particular, the lag time between blood collection and potassium determination was confined to a maximum of 30 minutes, and a positive correlation between platelet count and serum, but not plasma potassium concentration, was found. Other causes of this entity is from fist clenching and use of tourniquet as well.

With leukemia, many cases are now reported leading to false elevation of K.  If plasma and serum are separated quickly ( within 30 min), the normal K levels can be seen.  Potassium normally is now measured in heparinized tubes, so why does K still get elevated?  Lysis of cells can still occur.  Because of the high WBCS counts seen in some cases of CLL, spurious K can still be found. It is possible that its due to the impaired N/K pump in that state of elevated WBCs that contribute to the release of K from cells.  Trauma is the major cause. Drawing in a tube without shaking may help as well.

What about Stat ABG draws? The fragile WBCs are prone to mechanical stress frequently. The K measurements in many cases of CLL may be inaccurately elevated if sample of blood are not analyzed quickly and in absence of lysis inducing events. Arterial analysis done by ABG draws is quicker and perhaps less mechanical stress and may allow for more accurate K readings than venous draws.  In addition, perhaps it also doesn’t allow for that fist clenching and tourniquet use that might be leading to pseudohyperkelamia.

Is plasma K better than serum K?  A letter in NEJM in 1991 showed that elegantly that plasma K was superior to serum K in getting the more accurate K level in patients who had experienced trauma.

What is reverse pseudohyperkalemia? It is when the plasma K is higher than serum K and still is spurious in nature. Few cases reports have highlighted this entity as well. It is possible that this is due to a hiegtened sensitivity to heparin induced membrane damage in setting of a blood malignancy. 

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