Capillary blood sampling was first developed to extract blood samples from newborns and infants whose veins were too small for venous blood draws. The "heel stick" approach to collecting capillary blood samples from infants is still used in hospitals today. Decades later, the capillary sampling technique is more widely used. People with diabetes often measure their blood glucose at home using a "finger stick" method to collect capillary blood samples from their fingertip. Finger-stick sampling is also becoming more common in research and clinical settings – often to enable remote specimen collection for serology studies. For remote collection, the finger-stick sampling is self-managed by people at home without the need for professional assistance.
Capillary blood is drawn by using a small lancet to prick a person's finger (or upper arm, above the knee, etc.). The blood drop is then collected via a sampling tool like a Mitra® device.
The small, hand-held Mitra devices have an absorbent tip on the end. Based on VAMS® technology, the tip of the Mitra device absorbs the exact amount of blood needed for serology analysis. The sampled device is closed into its protective cartridge and placed in a sealed specimen bag, which fits inside a pre-stamped envelope. The envelope can be dropped in the nearest mailbox for sending to the lab. The blood sample on the device will dry in transit, and will be analyzed in the lab as a dried blood microsample.
Capillary vs. Venous Blood: What’s the Difference?
For the most part, capillary and venous blood can be used for the same serology applications without affecting the validity of the test results. There are some small differences in hemoglobin and hematocrit levels measured in each. These differences may vary in significance depending on things like a person's gender and the iron levels in their blood, for example.
Capillary blood also usually has significantly higher blood glucose levels than venous blood collected from the same person. However, tests comparing how well each type of blood performed when screening for antibodies for measles showed that both types were equally reliable for this purpose. Capillary blood samples have proven to be very useful in antibody studies of influenza and other viruses as well.
Using Capillary Blood for Serology Studies
Capillary blood’s proven reliability in antibody testing means the finger-stick method for dried blood sampling can be used to conduct serology studies of viruses, such as SARS-CoV-2.
Serology studies are one of the primary ways we investigate contagious diseases, to investigate the antibodies developed in response to viral infections. Serology using microsamples can tell scientists which antibodies are produced after a person is infected with a particular virus, as well as how long those antibodies might remain over time to protect the host from reinfection.
Serology blood testing has become more important than ever as a result of the Coronavirus Pandemic. Antibody testing was used to develop the COVID-19 vaccines that are providing populations with some protection against new infections from SARS-CoV-2. Scientists also use blood tests to identify and screen for SARS-CoV-2 variants as the virus mutates and evolves. Blood tests are also being used to develop booster shots that will help defend against variants of concern, when needed.
Capillary blood testing often can be used for remote specimen collection in place of phlebotomy services and in-clinic blood draws. The remote approach to specimen collection for lab testing helps to avoid exposure risks between researchers, clinical staff, and study participants, which is crucial to limiting the spread of SARS-CoV-2 and other pathogens. Remote blood collection has recently been used for a number of high-profile COVID-19 serology studies, including the National Institutes of Health (NIH) to track undetected cases of COVID-19, and recent efforts by Stanford University to study the impact of vaccination on a person's immunity and other body systems.
Benefits of Capillary Sampling
There are many reasons for researchers and clinicians to consider remote capillary blood sampling to replace venipuncture blood draws.
Capillary blood collection can be done at home with simple user-friendly devices like Mitra devices. The remote, self-managed approach is far more convenient for study participants who wish to avoid frequent visits to a blood draw center, or long commutes to medical centers. Offering remote options to participants or patients makes them more likely to adhere to recommended blood testing schedules.
Finger-Stick Sampling Benefits at a Glance:
Capillary blood can be drawn from many different sites on the body, allowing collection sites to be rotated to reduce the likelihood of sensitivity or scarring.
Capillary blood draws take tiny amounts of blood. Not only does this help alleviate anxiety for squeamish study subjects, it also reduces the need for risky high-volume blood draws in small children or the elderly.
Finger-stick sampling is much less intimidating and painful than venous sampling. This makes it a better choice for collecting blood from children and people with needle anxiety or those with cognitive disabilities.
Finger-stick sampling can be performed virtually anywhere, making it a good choice for sampling athletes in the field, study subjects in remote areas, etc.
Unlike liquid blood samples (which must be shipped and stored at specific temperatures) or plasma samples (which are viable at room temperature, and require a centrifuge to obtain), capillary blood samples can be shipped and stored as dried samples after remote specimen collection using a remote sample collection kit. This allows the samples to be shipped, stored, and handled without refrigeration or other special considerations.
Limitations of Capillary Sample Collection
Like any method of sample collection, capillary sampling has its limitations. Here are some things to consider when opting for capillary sample collection:
Blood cells sometimes rupture during capillary blood collection, which may affect study results.
Capillary blood sampling is not suited for every type of testing. Researchers and physicians must be certain the tests they want to run will produce reliable results with capillary samples.
If capillary blood is drawn from the same puncture site too many times, the area may become overly sensitive, or may scar.
End-users may still feel faint at the sight of blood, or may bleed too much, so a family member can assist with collection, and help apply a bandage and pressure to the sampling site after the event.
End-users can develop an infection at the sample site if they don't wash the sampling area and keep the lancet and sampling supplies sterile.
Some people (especially infants) develop lesions called “calcified nodules” at the site where the lancet pierced their skin. These are rarely serious and will usually go away over time.
Overall, capillary blood sampling with finger-stick microsampling methods are promising alternatives to venous blood draws. The two methods are not always interchangeable, and their strengths and weaknesses must be taken into account in each use case. However, when capillary blood is used for a suitable application, it offers a range of benefits that make it an appealing choice for all parties involved.