Venous blood samples are commonly used in clinical diagnostics. However, for some situations, capillary blood samples are preferred. For example, the small blood volume facilitates sample handle in a near-patient setting, and a finger-stick is easier on the patient. Capillary blood sampling is also an option for patients that are hard to puncture such as elderly and infants. Additionally, capillary samples are suitable for pre-donor testing in blood bank applications, as it saves the vein for donation.
Top tips from the laboratory technician
When performing a finger stick, there are some things to consider that can affect the hematology test results. Here, Anette Lindström Laboratory Engineer at Boule Diagnostics, shares her best practices for how to draw blood to ensure reliable results with the capillary inlet of Swelab Alfa Plusand Medonic M32 hematology analyzers.
Swelab Alfa Plus and Medonic M32 allow for analysis of both venous and capillary samples. With the micro-pipette adapter (MPA) inlet, you can get a complete blood count in about one minute from just one drop of blood.
Performing a fingerstick: what can affect the results?
Due to PLT adhesion to tissue and capillary walls and imprecision in preparation and blood draw procedures, discrepancies between capillary and venous blood values might occur for some parameters. For example, platelets might be 5%–10% lower in capillary blood and PLT clumping can affect the lymphocyte count, resulting in falsely elevated white blood cells.
Swelab Alfa Plus and Medonic M32 are equipped with a high-precision shear valve that provides an absolute volume for the cell count from the open tube, cap piercing, and pre-dilute inlets. As the MPA inlet bypasses the shear valve, it is of utmost importance to ensure correct volume is drawn by making sure the whole capillary is filled with blood and by wiping of any excess blood outside of the capillary before sliding it into the MPA module.
Blood collection procedure
To ensure reliable results with the MPA inlet, the following blood collection procedure can be used.
- Warm the site for skin puncture 3–5 min with a heating pad or under warm water to increase the blood flow. Note! Make sure heating pad or water does not hold a temperature above 42˚C.
- Disinfect site and let air dry.
- Remove the MPA adapter from the analyzer by gently pulling the handle.
- Perform the puncture: a. Take a firm grip of the whole hand, not just one finger.
b. A deep and firm puncture allows obtaining free-flowing blood drops, decreasing incorrect or non-reproducible results. Note! Preferably use 2 mm lancet. c. To get a good flow, hold the hand in hanging position. Note! Only add a light pressure, followed by release to obtain a free flow and to avoid getting tissue fluid in blood sample, which would result in falsely low cell counts.
- Wipe away the first drop of blood with a clean tissue.
- When second drop forms, collect in micropipette. Note! Fill the micropipette completely with fresh whole blood and carefully wipe off excessive blood on the outside surface.
- Insert filled capillary into the MPA adapter, which is inserted into the analyzer, and analysis cycle will automatically begin. Note! Samples should be analyzed directly after collection, and for optimal results, no longer than 10 min from collection.
Capillary sampling for the MPA inlet.