LabNotes® - Volume 17, No. 1, 2007

Differences in Composition of Capillary and Venous Blood Specimen

Blood obtained through skin puncture (capillary blood) differs from blood that is obtained through venipuncture. When analyzing the major characteristics of capillary blood, such as pH, PCO₂, PO₂ and oxygen saturation, freely flowing capillary blood is actually more similar to arterial than to venous blood. On the other hand, due to the method of collection, capillary blood is contaminated with interstitial and intracellular fluids, which will influence the analytic values obtained from these samples. It is for this reason that capillary blood is not recommended for coagulation testing. Differences also exist between venous and capillary blood analyte concentrations (see table below). This is by no means an exhaustive list, and since the data were taken from two different studies, does not include identical analytes for both specimen types, i.e. plasma and serum.

SPECIMEN		Capillary Value Greater Than Venous Value (%)		No Difference between Capillary and Venous Values		Capillary Value Less Than Venous Value
SERUM1		Glucose 1.4 Potassium 0.9		Phosphate Urea		Bilirubin 5.0 Calcium 4.6 Chloride 1.8 Sodium 2.3 Total Protein 3.3
PLASMA2		Alanine Aminotransferase 32.8 Albumin 5.4 Amylase 19.6 Aspartate Aminotransferase 5.0 Calcium 1.2 Chloride 4.5 Creatinine Kinase 24.6 Total Protein 13.2		Glutanyl Transferase		Alanine Phosphatase 5.4 Creatinine 8.3 Potassium 2.6 Sodium 0.7 Urea Nitrogen 24.0 UricAcid 2.5
These differences between capillary and venous samples are important for laboratorians to keep in mind because the analyte reference ranges for one may not be reliably applied to the other. In order to insure the quality and accuracy of their results, laboratories should determine separate capillary and venous analyte reference ranges where applicable.

Footnotes:

1 From. Kupke 1R, Kather B. Zeugner S. On the composition of Capillary and Venous Blood Serum. ClIN, CHIM, ACT, 1981; 112; 177-85.

2 From Rommel K, Koch C-D, Spilker D. Einfuss Der Material Ewinniung AUF Clinisch-Chemische Paarameter in Blut, Plasma
UND Serum BEI PATHEN MIT STABILEM AND ZENTRALISIERTEM KREISLAUF. J CLIN CHEM BIOCHEM 1978,16:373-80.