Hi, I’m Matt Binnicker, the Director of Clinical Virology and Vice Chair of Practice
in the Department of Laboratory Medicine and Pathology at Mayo Clinic.
Measuring potassium levels is common in patient management, especially during the evaluation
of electrolyte balance, cardiac arrhythmia, muscle weakness and renal failure.
But did you know that the type of blood sample that is tested can have an important impact
on the potassium results that are obtained?
In this month’s "Hot Topic," my colleague Dr. Brad Karon, will provide you with an overview
of the potential differences in potassium levels that may occur when testing capillary
blood versus a venous sample.
I hope you enjoy this month’s "Hot Topic," and I want to personally thank you for allowing
Mayo Clinic the opportunity to be a partner in your patients’ health care.
Thank you, Matt.
For those of you who have not attended our phlebotomy conference here in Rochester, the
title of my “Hot Topic” today, “Phlebotomy Top Gun,” may seem a little odd, but I hope
I’ll be able to explain to you what “Phlebotomy Top Gun” is and what we try to do at our
annual phlebotomy conference.
Every year at our phlebotomy conference, I present a talk called, “Phlebotomy Top Gun.”
The format for “Phlebotomy Top Gun” is a case-based presentation.
I solicit from conference attendees ideas for cases based upon issues or questions you
have related to phlebotomy.
I present these as case-based scenarios, and using an audience response voting mechanism,
the attendees of the conference vote on the action or answer they feel is appropriate
for each case.
I then present the evidence and data from the literature and Mayo collective experience
related to the topic.
At the end of the case, the attendees vote again.
For each case, I can see whether I’ve been able to change anyone’s mind on the question
at issue by presenting the data and information relevant to the topic.
This is an actual case from a previous year’s phlebotomy conference “Phlebotomy Top Gun”
presentation, derived from a question about the validity of measuring potassium from capillary
puncture blood samples.
I have no disclosures related to today’s talk.
Based upon the question submitted by a phlebotomy conference attendee two years ago, I presented
this question to the conference audience.
Regarding capillary compared to venous potassium (K) measurement, which of the following are
true statements: 1. Capillary samples are, on average, more hemolyzed
than venous samples; therefore, potassium is systematically higher in capillary samples.
2. For a given amount of free hemoglobin (hemolysis), the increase in potassium is higher in a capillary
sample compared to a venous sample.
3. Experienced techs or phlebotomists can collect capillary samples with no more free hemoglobin
(hemolysis) than a venous sample.
4. Both 1 and 2 are true. The first and second responses are true.
Or, 5. All of the above responses are true.
At the phlebotomy conference, we would use a live audience feedback system to poll the audience and see
what answers audience members believed to be correct about this issue.
For those of you watching now, which do you think is the correct answer?
What I do next with each case in the “Phlebotomy Top Gun” presentation is go through data
and evidence, either internal or external to the Mayo Clinic practice, that I can find
related to this question.
At the end of each case, I re-poll the audience to see if anybody’s mind has been changed.
To remind our audience today, capillary “blood” is really a mixture of arterial blood, venous
blood, and interstitial fluid (the fluid that bathes cells within our tissues), and fluid
released from cells during the capillary puncture itself.
The combination of interstitial fluid and cellular fluid is often called “tissue fluid.”
While concentrations of most chemical analytes are identical or very similar between arterial
and venous blood, it is the tissue fluid that can vary dramatically in the concentration
of various analytes we measure in the laboratory.
To make matters more complicated, each capillary puncture will have a different amount of tissue
fluid collected into the sample.
The importance of this variable amount of tissue fluid collected in a capillary sampling
differs by analyte and depends upon how different cellular and interstitial concentrations of
the measured analyte are compared to venous or arterial blood.
For example, there is no hemoglobin in interstitial fluid, so variable amounts of interstitial
fluid will cause variability in capillary hemoglobin measurement compared to venous
or plasma hemoglobin.
After meals, interstitial fluid glucose concentration changes more slowly than venous or arterial
glucose, and this complicates interpretation of capillary glucose in the non-fasting state.
So, what about capillary sampling for potassium measurement?
Let’s explore this question further, remembering that we break more cells during capillary
puncture than during venipuncture.
On this slide, I provide a reference to an older study comparing potassium values from
infants with simultaneous blood collection from an in-dwelling arterial catheter compared
to a capillary heel stick puncture.
The capillary samples yielded potassium measurements that were on average 1.2 millimolar higher
than the paired arterial serum sample, consistent with the results of several other studies
that demonstrated that capillary potassium measurements do run higher than values obtained
from venous or arterial samples.
But the question is, “Why is this?”
This next study, also a bit older, compared the average amount of free hemoglobin, from
hemolysis during capillary blood collection between infants and children of varying ages.
For comparison, the average amount of free hemoglobin in a venous specimen collected
from an adult is less than 50 mg/dL.
As you can see, capillary punctures performed on infants in the first two weeks of life
resulted in, on average, almost 400 mg/dL of free hemoglobin in these samples.
By two years of life, the average free hemoglobin level decreases to around 150 mg/dL.
So, capillary puncture in very young infants will have more free hemoglobin than capillary
puncture in children older than two years of age.
This next study was performed at Mayo Clinic around ten years ago.
We wanted to know whether phlebotomist training impacted the amount of hemolysis, or free
hemoglobin, in capillary puncture samples from newborns.
We looked at three groups of capillary puncture samples.
Group One consisted of capillary punctures performed by an outpatient phlebotomy group,
while Group Two were samples collected by the very same group of outpatient phlebotomists
after specific training or retraining (in this case) on capillary puncture in infants.
Group Three consisted of samples collected by an inpatient pediatric phlebotomy group
that has more experience and does more newborn capillary collections.
As you can see, the bottom line of our study is that experience in terms of number of capillary
punctures or re-training staff has no effect on the hemolysis levels or amount of free
hemoglobin in capillary puncture samples from healthy newborns.
The last formal study that I’ll discuss is perhaps the most interesting.
This study compared the amount of free hemoglobin, this time measured as a semi-quantitative
index, to average potassium concentrations in both capillary and venous samples.
This study compared average potassium levels in thousands of capillary and venous samples.
The average potassium capillary and venous samples that had no hemolysis was the same
as we would expect, at 4.0 millimolar.
A small amount of free hemoglobin in the venous sample, the semi-quantitative H indices of
1 or 2, resulted in modest increases in potassium to 4.2 to 4.5 millimolar, respectively.
In contrast, the same amount of free hemoglobin increased potassium in capillary samples to
4.5 and 5.0 millimolar.
In other words, the same amount of hemolysis increased potassium concentration more in
capillary samples compared to venous samples.
This last slide describes our internal validation study of capillary blood gas testing at Mayo.
We collected capillary blood gas specimens from 22 healthy infants, after heal warming
and with hand transport of the capillary blood gas specimens to the testing lab.
The average potassium level in the samples was 5.7 millimolar, so our study was consistent
with others showing on average higher potassium levels in capillary samples compared to the
venous samples, which have an average around 4.0.
A look at the individual values for the 22 infants demonstrated potassium levels in these normal
healthy infants of anywhere between 4 and 10 millimolar.
Based upon this data showing wide variability in potassium in healthy infants, we decided
not to report potassium values in our capillary blood gas samples from newborns.
To summarize the data and studies, capillary samples contain more free hemoglobin, higher
levels of hemolysis, than do venous or arterial blood samples.
One study did find that for a given amount of free hemolysis or free hemoglobin, potassium
increases more in capillary samples compared to venous samples.
I believe that potassium measurement in capillary blood samples should only be done in laboratories
that have the ability to measure free hemoglobin with every measurement and have a good process
to withhold results when there is too much free hemoglobin to produce reliable potassium
results.
Testing laboratories might also consider implementing a stricter or different threshold for the
amount of hemolysis or free hemoglobin in capillary samples.
At this point during the Top Gun phlebotomy presentation at our phlebotomy conference,
I would re-poll the audience to see if the attendees had changed their answers based
upon the information I had provided.
I would also reveal what I think is the correct answer—in this case, I believe the correct
answer is #4, the first two statements are true.
Capillary samples have more free hemoglobin than venous samples, and there is some evidence
that a given amount of hemolysis in a capillary sample increases potassium more than the same
amount of hemolysis in a venous sample.
However, there is no evidence that phlebotomist training or experience can impact hemolysis
levels in capillary punctures.
I have provided for you references to the studies I mentioned here today.
Thank you for your time and attention.
If you liked today’s presentation, or even if you didn’t, please do consider attending
the upcoming Mayo Medical Laboratories phlebotomy conference, “Phlebotomy 2018: Building a
Resilient Culture,” here in Rochester, Minnesota, on March 22 and 23 in 2018.
As in previous years, the conference will feature presentations from expert speakers
covering a variety of topics related to phlebotomy skills, phlebotomy management, and other topics.
Learning will take place via both large groups and small breakout sessions.
Tours of Mayo Clinic facilities will also be offered during the conference.
I will also be presenting an all new “Phlebotomy Top Gun” presentation based on this year’s
attendees’ interests, issues, and concerns during the registration process.
More information on the phlebotomy conference program can be found on the
Mayo Medical Laboratories website.
Thank you for listening and have a wonderful day.