Not All Blood Is Created Equal

Hello, my friends,

First, an apology for missing last week’s post. I was traveling to Virginia for the ASCP Virginia Chapter Phlebotomy Seminar, where I had the privilege of speaking — and unfortunately, the hotel had other plans when it came to internet access.

It was an incredible experience. To the organizers who put the seminar together and to everyone who attended — thank you. Virginia, it turns out, is home to a remarkable concentration of people who have made lasting contributions to the phlebotomy and laboratory world, and I feel lucky to have finally met some of them in person. I left inspired, and I hope it won’t be my last time there.

Early in my hospital career, I had orders to collect blood samples from a newborn who had been brought to the ED. I was running behind, and by the time I entered the infant’s room a nurse had decided she didn’t want to wait for me and she was going to attempt to collect the blood herself. I walked into the room to find the nurse preparing to place a peripheral IV on the infant. The doctor had ordered a bilirubin and a CBC — two tests, both routine, both very doable from a heel stick. I asked if she’d considered capillary collection instead. She paused, looked at the baby, looked at me, and said, “Oh. Yeah. I always forget we can do that.”

We did the heel stick. The baby cried for about thirty seconds. No IV, no catheter dwelling in a tiny vein, no tape on a fragile arm. Just a small lancet wound on the lateral heel, two microcollection tubes. Done. The infant ended up being okay, just a fussy newborn with a mild fever and nervous parents.

That moment stuck with me — not because the nurse was wrong or incompetent, but because it revealed something true about how we sometimes default to the most invasive option out of habit. That infant didn’t need an IV, or a venipuncture for two small samples. Why not see what the labs say before placing a potentially unnecessary IV in a tiny newborn arm? The more invasive option is still available if needed. The nurse didn’t think of it, and she didn’t know how to perform it properly, but once I reminded her of the option, she was all for assisting me in trying that first.

That being said, capillary collection is its own skill set, with its own indications, its own limitations, and its own quirks. And one of the biggest quirks is this: the numbers don’t always match venous results. Sometimes it’s a great option, sometimes it’s better to obtain venous blood — not because someone made a mistake, but because capillary and venous blood are genuinely different things.

They’re Not the Same Blood

This surprises a lot of people, including experienced healthcare providers. We tend to think of blood as blood — same patient, same body, same values. But what comes out of a fingerstick or a heel stick isn’t purely venous or purely arterial. Capillary blood is a mixture: arterial blood delivered through the capillary bed, venous blood returning from the tissues, and small amounts of interstitial and intracellular fluid that inevitably get drawn in during collection.

That mixture, under ideal collection conditions, tends to be closer to arterial blood in composition — which is why warmed capillary specimens are sometimes called “arterialized.” But ideal conditions don’t always happen. Technique errors, patient factors, and the basic physiology of the capillary bed can shift that composition significantly. The result is a specimen that looks a lot like venous blood in some analytes and nothing like it in others.

Understanding which analytes behave which way — and why — is the difference between a phlebotomist who just collects the specimen and one who understands what they’re actually collecting.

The Analytes That Don’t Always Agree

Glucose

Glucose is the analyte most people already know behaves differently by collection method, but they don’t always know why or by how much.

In a fasting state, capillary and venous glucose values are fairly close — usually within a few mg/dL of each other. But after a meal, the arteriovenous difference opens up significantly. Capillary glucose can run 20–70 mg/dL higher than venous glucose in the postprandial state because glucose is being actively delivered to the tissues via the arterial circulation and hasn’t yet been extracted. A capillary draw catches that glucose before the tissues have had their cut.

This matters clinically in a few ways. First, most point-of-care glucometers are calibrated for capillary whole blood — using a venous sample on a capillary-calibrated device can produce inaccurate results, and some analyzers will flag this. Second, a capillary glucose and a venous glucose drawn close together on the same patient may look like a discrepancy when they’re actually both correct. The collection method needs to be documented.

Hemoglobin A1c

A1c is worth addressing specifically because it’s increasingly being run on point-of-care analyzers from fingerstick samples. For most patients, capillary A1c correlates well with venous A1c — the hemoglobin being measured has been circulating for weeks, so the arteriovenous mixing effect matters less than it does for real-time analytes like glucose.

The complication is technique. Hemolysis and specimen dilution from poor capillary collection technique can affect A1c results, particularly on HPLC-based analyzers. Some POC A1c platforms have been validated specifically for capillary collection; others have not. If your facility is running A1c from a fingerstick, it’s worth knowing which platform you’re using and whether capillary collection has been included in the method validation.

Potassium

This is the one that makes laboratory professionals nervous, and for good reason.

Capillary potassium runs higher than venous potassium, and the gap can be clinically significant. There are a few reasons for this. Tissue fluid contamination during collection introduces intracellular potassium from disrupted cells. Milking the puncture site — squeezing the finger or heel repeatedly to force blood flow — causes mechanical hemolysis that releases even more intracellular potassium. Cold extremities slow blood flow and increase the likelihood of stasis, which also artificially elevates potassium.

The problem is that a falsely elevated potassium can look like true hyperkalemia. A patient who appears to have a potassium of 6.2 from a fingerstick may have a venous value of 4.8. That false result can trigger unnecessary interventions — cardiac monitoring, medication changes, repeat testing — based on a number that was never real.

If potassium is the reason for the draw, or if the result will drive a clinical decision, capillary collection is generally the wrong choice.

Hematocrit and Hemoglobin

Capillary hematocrit tends to run higher than venous hematocrit, particularly in neonates. There are a few reasons: tissue fluid dilution is smaller relative to the total sample in a well-collected capillary draw, and the arterial component of capillary blood has a higher oxygen saturation and slightly different cellular composition than venous blood.

In newborn screening and neonatal bilirubin workups, this difference is generally understood and accounted for by the reference ranges used. But if a capillary CBC hematocrit is being used to evaluate anemia or polycythemia in an infant, the collection method should be clearly documented and the ordering provider should know the specimen source.

Prothrombin Time (PT/INR)

POC coagulation testing — particularly INR monitoring for patients on warfarin — is increasingly done from fingerstick samples, and this is an area where the evidence is more nuanced than most people realize.

Capillary INR and venous INR generally correlate well in stable, therapeutic patients. Studies have generally found that for patients who are in a steady anticoagulation state, POC fingerstick INR is a clinically acceptable alternative to venous lab INR. However, the correlation weakens at the extremes of the therapeutic range — very high or very low INR values — and in patients who are acutely ill, in shock, or have poor peripheral perfusion. In those situations, the capillary result may not accurately reflect the patient’s true coagulation status.

Again, the key variable is knowing when capillary is appropriate versus when you need a venous draw.

Bilirubin

Transcutaneous and capillary bilirubin measurements in neonates are workhorses of the newborn nursery. For routine hyperbilirubinemia screening in otherwise healthy term neonates, capillary bilirubin is generally reliable and widely used.

Technique matters here more than in almost any other analyte. Specimen exposure to light causes photo-oxidation of bilirubin, which means the tube needs to be protected from light from the moment of collection. A slow fill, a cold heel, and excessive milking can all affect the result. And because bilirubin is one of the primary drivers of clinical decision-making in the first days of life — including decisions about phototherapy — accuracy is non-negotiable.

When Capillary Collection Is the Right Call

None of this is an argument against capillary collection. It has legitimate, well-established indications, and in the right situation, it’s not just acceptable — it’s the better choice.

The clearest example is neonates, because the clinical math is more complicated than it might seem.

When a physician orders a bilirubin and a CBC on an infant, the instinctive response in many clinical settings is to reach for venipuncture supplies. I’ve seen it happen dozens of times. But before going straight to a peripheral blood draw, it’s worth asking: can capillary collection get us what we need?

For a bilirubin and a CBC — two of the most common neonatal orders — the answer is often yes. A proper heel stick using an automated lancet on a warmed heel, collected into microtainers, can yield adequate volume for both tests without puncturing a fragile vein. That matters for a few reasons.

The evidence on pain is more nuanced than you might expect. Several studies have found that skilled venipuncture in newborns is actually less painful than heel stick — particularly when milking is required to get adequate volume. A Cochrane review has suggested venipuncture may be the procedure of choice in term neonates in some contexts. However, the real-world consideration that often tips the balance is this: the heel stick preserves venous access. If the infant ends up needing an IV later — for fluids, antibiotics, or a longer stay — that vein is still there, unused and undamaged. Performing a venipuncture you may not need, on a baby who may go home tomorrow, is a different risk-benefit calculation than placing an IV on an infant who is clearly ill.

That said, venipuncture in a newborn is not without its own risks, and those risks deserve weight in the decision. Neonatal anatomy is unforgiving — veins are small, fragile, and frequently difficult to visualize, and the structures surrounding them are compressed into a much smaller space than in older patients. Nerves, tendons, and arteries run in close proximity to common venipuncture sites, and a newborn cannot tell you that the needle is in the wrong place. Unlike an adult who flinches or says “that burns,” a neonate’s only feedback is crying — which is already happening.

Hematoma formation is a real concern, particularly in premature or jaundiced infants where skin and vessel integrity may already be compromised. Repeated failed attempts cause cumulative tissue trauma, and in a patient whose total blood volume may be as little as 80–90 mL/kg, even minor blood loss from multiple punctures is not trivial. A skilled phlebotomist can do their best to perform a safe venipuncture on a newborn, but those risks will still be there. The heel stick may be the better choice precisely because while it may be more painful, it avoids the risks that come with venipuncture, whether it’s a routine blood draw or IV placement.

This is the conversation worth having at the bedside. Not “heel sticks are always better” — they’re not — but “do we actually need an IV right now, or can capillary collection get us through this first set of labs?” When the answer is yes, the heel stick spares the infant from an invasive procedure that may have been unnecessary.

Beyond neonates, appropriate indications for capillary collection include patients with severely compromised venous access due to obesity, chemotherapy-damaged veins, burns, or scarring; patients for whom multiple venipuncture attempts would be more traumatic than a fingerstick; and POC testing scenarios where capillary collection has been validated for the specific analyte and device in use.

The Errors That Make Everything Worse

Capillary collection has inherent method differences from venous collection — but poor technique amplifies those differences significantly. The errors that cause the most trouble:

Milking. Squeezing or “milking” the puncture site to force blood flow is probably the single most damaging technique error in capillary collection. It introduces tissue fluid, causes mechanical hemolysis, and elevates potassium, glucose, and other analytes. If you are doing anything beyond a gentle squeeze to get adequate blood volume, then perhaps the puncture was inadequate — try a deeper lancet (not more than 2mm), a better-placed stick, or on a warmed site.

Skipping the warm. Warming the collection site for 3–5 minutes increases blood flow up to sevenfold and produces a much more arterialized specimen. Cold fingertips and cold heels produce sluggish, poorly arterialized blood that requires milking to collect and yields results that look more like venous blood than they should.

Not wiping the first drop. The first drop of blood contains tissue fluid, residual alcohol from skin prep, and cellular debris from the puncture itself. It must be wiped away. This is non-negotiable and yet it gets skipped regularly, especially when volume is tight and every drop feels precious.

Slow fill and clotting. Microcollection tubes need to fill promptly and be mixed as they fill. A slow fill from a cold or inadequately punctured site allows partial clotting to begin, which affects CBC results and coagulation testing. Remember the order of draw for microtainer tubes is EDTA, other additives, and then serum or no additive tubes last.

Document the Method. Every Time.

This is the part of capillary collection that has the least glamour and the most practical importance: if you collect capillary blood, document it.

The ordering provider may not know the specimen source unless you tell them. Reference ranges for some analytes are method-dependent. Clinical decisions — including medication adjustments, transfusion thresholds, and phototherapy initiation — can hinge on whether a result came from a heel stick or a venipuncture. If the method isn’t documented, a clinician comparing a capillary result to a previous venous result may conclude there’s an unexplained discrepancy when there isn’t one.

It’s a five-second notation that can prevent a cascade of unnecessary follow-up.

Back to the Beginning

The nurse who forgot about the heel stick wasn’t negligent. She was busy, she was trained to place IVs, and placing an IV is what she defaulted to when the order came in. What she needed — what a lot of bedside nurses and even some phlebotomists need — is someone with enough capillary collection knowledge to say: we have another option here, and here’s why it might be better.

That’s not a small thing. In neonates especially, every unnecessary procedure carries weight. Every undisturbed vein is a resource conserved. And every phlebotomist who understands not just how to do a heel stick, but when it’s appropriate and what its results actually mean, is adding something to that patient’s care that no one else in the room may be equipped to offer.

Same patient. Different numbers. Know why.

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