capillary blood vs. venous plasma: how close is microsampling to the gold standard?

Although there are specific situations where plasma derived from venous blood is not the best option for diagnosis, treatment, or therapeutic drug monitoring (TDM), it remains the widely accepted "gold standard" in clinical practice.

For instance, certain tests, such as blood gas analysis, are more effectively performed using arterial or capillary blood specimens.

Nevertheless, venous plasma continues to be the primary sample type for most laboratory-based testing.

However, recent advances in microsampling technology are shifting this paradigm.

With the growing number of advantages offered by remote blood collection, the question is no longer "Which sample type is best?" but rather, "How close to the best are test results obtained from capillary blood using microsampling techniques?"

The Biological and Chemical Composition Differences Between Capillary and Venous Blood

Understanding the differences between capillary and venous blood is important for their use in diagnostics. Although both types of samples contain the same fundamental components—plasma, red and white blood cells, and platelets—their compositions vary slightly because of their anatomical origins.

Capillary blood is a mixture of arterial, venous, and capillary blood, along with interstitial and intracellular fluids. As a result, the composition of capillary plasma may exhibit the following characteristics:

• Higher concentrations of proteins, calcium, and chloride
• Lower concentrations of potassium, sodium, and urea nitrogen
• A closer match to arterial blood when measuring pH and blood gases

These biological and chemical differences can impact the performance of assays. Therefore, establishing a correlation between the results of capillary and venous samples is essential for ensuring data accuracy.

Microsampling and Diagnostic Accuracy

When analytical concordance is confirmed, capillary blood samples collected through microsampling can provide high-quality and reliable results. Several studies support this conclusion:

A study published in the European Journal of Clinical Pharmacology found that measuring capillary piperaquine levels may be useful for field assessments of malaria treatment efficacy.

Additionally, research in the American Journal of Clinical Pathology demonstrated the effectiveness of point-of-care (POC) capillary blood testing for monitoring the international normalized ratio (INR).

These examples illustrate that microsampling can serve as a robust alternative to venous blood draws in both field and clinical settings, especially when remote collection is necessary or when venipuncture is impractical.

Best Practices for Obtaining High-Quality Capillary Blood Samples

To ensure the accuracy of test results, proper sample collection and handling are essential, regardless of the sample type. Follow these guidelines for reliable capillary blood sampling:

1. Ensure the patient is positively identified.
2. Choose the appropriate puncture site and collection device based on the test requirements.
3. Warm the puncture site beforehand to promote blood flow.
4. Thoroughly disinfect the puncture site.
5. Discard the first drop of blood to prevent contamination.
6. Collect the sample quickly, avoiding any milking or scraping.
7. Properly label and store samples to maintain their integrity.

Final Thoughts

Capillary microsampling offers compelling benefits over venipuncture—particularly in remote, pediatric, or repeat sampling scenarios.

While it may not always replace venous plasma in every application, it can serve as a clinically valid alternative in many.

By maintaining rigorous quality assurance practices and understanding the biological and chemical composition differences between capillary and venous blood, laboratories and clinicians can confidently adopt microsampling where appropriate.

The future of diagnostics may not depend on a single gold standard, but rather on selecting the right tool for each patient and use case.