Specimen collection & handling issues It is not uncommon for us to get a high Potassium result, but on re-bleeding, it is within the normal range. Our laboratories have panic levels for certain analytes and for Potassium it is > 6.2 mmol/l. Above this level we are obliged to phone the ward/surgery/doctor to inform them. After hours this can get complex as if your surgery is closed, GP assist is commonly the next avenue. There have been situations where the patient is hospitalised on the strength of the high potassium, only to find it was normal on re-bleed.
The first things in the lab that we check are:
- Haemolysis: Rupture of the red cells is mostly caused by squirting the blood into the collection tube through the needle, or by a traumatic draw.
- Old sample: When the sample is old, the potassium pump fails to pump potassium into the red cells, so it leaks out into the serum. It is very handy if the samples have a date and time on them to check this, otherwise it can take a few phone calls to get the information.
Things that we can’t check are: - Refrigerated sample: Cold can also inhibit the glycolysis which provides energy for pumping potassium into the cell, so if a blood sample is put in the fridge or < 15 degrees for more than 2 hours, falsely elevated potassium’s can result.
- Vigorously shaking the tubes: This causes mechanical rupture of the cells. - Order of draw: Lavender top potassium EDTA tubes or grey top sodium fluoride tube drawn before the serum chemistry tube. Some of the potassium anticoagulant can carry over into the chemistry tube.
- Mislabelled specimen: Taken from the wrong patient.
- Leaving tourniquet on for an extended period. : Affects water balance & red cells & platelets rupture releasing potassium.
- Excessive fist clenching: Repeated fist clenching with or without the tourniquet causes potassium to be released from monocytes into extracellular fluid during muscle contraction. One study showed the incidence of hyperkalaemia is lowest in summer. The reason for this may be multifactorial, but patients with difficult veins can be worse in winter when they come in cold and pump their arms to get the veins to appear. Avoid the fist clenching.
The clinical consequences of a potassium above 6.2 is a disturbance in cardiac rhythm, so our process of phoning must be adhered to so as not to miss the genuine cases. If our staff have collected the sample, and we get a high potassium that is not expected, we will endeavour to get a recollect if the sample is visibly haemolysed and the patient is readily accessible. The list above is by no means complete. If you have any question, feel free to discuss them with Dr Roberts or our duty Biochemist.