The short answer
In short: for most routine parameters, carefully collected capillary blood delivers results that agree well with a venous blood draw. A growing number of studies show this, including a meta-analysis of 26 comparison studies [1]. Some parameters, however, are sensitive to how the sample is collected, how much blood is obtained or how it is transported. For these, capillary blood is the wrong sample type.
This article summarises the research on self-collected liquid capillary blood: where agreement with venous blood is good, where it reaches its limits, and what role collection technique and shipping play. The numbers in square brackets refer to the reference list at the end of the page.
Key points
- In a meta-analysis of 26 studies, lab values from self-collected capillary blood agreed well with venous blood (pooled correlation r = 0.89). When results were classified as normal or abnormal, agreement was 0.99. [1]
- In a study of 105 participants, people collected their own capillary blood with Tasso+ and sent it to the laboratory by standard post. HbA1c, creatinine and blood lipids agreed closely with venous samples delivered to the laboratory by courier (concordance correlation above 0.95, 0.999 for HbA1c). [2]
- Of 34 routine clinical chemistry parameters, 30 of the 33 assessable parameters met the quality criteria for allowable deviation from venous blood. [4]
- Thyroid values (TSH, free T4) and sex hormones (including testosterone, estradiol, LH, FSH and prolactin) showed correlations of 0.97 to 1.0 with venous values in comparison studies. [7, 9]
- Most of 47 routine markers studied remained stable in whole blood for up to 72 hours at room temperature. Potassium, phosphate and iron were unstable, and heat (37 °C) reduced the stability of further markers. [3]
- Not every value is suitable: platelets, potassium and glucose are among the parameters for which capillary blood performs worse in studies. [4, 6, 12, 13]
Capillary and venous blood: what is the difference?
In a conventional blood draw, venous blood is taken from a vein in the arm with a needle. Capillary blood comes from the fine vessels just beneath the skin. It is a mixture of arterial and venous blood with a small proportion of tissue fluid.
The main sources of error lie in the collection itself. If the puncture site is squeezed to obtain more blood, red blood cells can break down (haemolysis) and tissue fluid dilutes the sample. Sample volumes that are too small also put the analysis at risk [19, 20]. An editorial therefore stresses that validation studies need to reflect these pre-analytical factors and real-world rejection rates [19].
Vacuum-assisted upper-arm devices such as Tasso+ or TAP Micro Select collect the blood straight into a tube and avoid squeezing. With fingertip collection, values vary more from drop to drop: across consecutive drops, the coefficient of variation was up to 3.4 times higher for haemoglobin and up to 5.7 times higher for leucocytes than in venous samples [20]. Only from a pooled volume of 60 to 100 µl did the variation fall within the range of the analyser’s own variability [20].
Note: This article covers liquid capillary blood. Dried blood spots (DBS) are a separate method; the findings summarised here apply to liquid capillary samples.
What the research shows
One point up front: the studies cited examined specific collection devices, laboratory methods and study settings. They did not test Probatix kits or Probatix partner laboratories, and their results can only be transferred to comparable conditions.
The overall picture
A systematic review with meta-analysis evaluated 26 studies in which participants collected their own capillary blood and the results were compared with a venous draw [1]. Across all lab values, the pooled correlation was r = 0.89 (95% confidence interval 0.86–0.92) [1]. Results varied considerably between the individual studies, however (heterogeneity I² = 94.2%) [1]. When results were only classified as normal or abnormal, agreement was 0.99 (95% confidence interval 0.98–1.00) [1]. Self-collection was perceived as less painful than venepuncture, and upper-arm devices as less painful than the fingertip [1].
For day-to-day laboratory work, it also matters how samples from self-sampling devices fit into automated laboratory workflows. A review from Leiden University Medical Center describes challenges such as cost, transport regulations and sample volume, and makes recommendations on robust device design and sample quality [21].
Clinical chemistry and metabolism
In a study of 105 participants, people collected their own capillary blood with Tasso+ at their GP practice and sent the sample to the laboratory by standard post; the venous comparison sample went by courier [2]. For HbA1c, the relative bias was 0.46% and the concordance correlation coefficient (CCC) 0.999; for creatinine, 4.8% and 0.972 [2]. For LDL, HDL and total cholesterol, the relative bias was between 3.0 and 5.7%, with a CCC above 0.95 in each case [2].
Another study compared fingertip capillary blood with venous blood for 34 routine clinical chemistry analytes [4]. Of the 33 analytes with total allowable error (TEa) criteria, 30 met these criteria [4]. Of the 29 analytes with CLIA criteria (2024 version), only glucose failed to meet the requirements [4].
A study of 480 adults and 960 capillary samples compared two collection sites, the fingertip and the upper-arm device TAP Micro Select, for 18 analytes including thyroid values [5]. 16 of the 18 analytes were clinically comparable; potassium and chloride were only interchangeable to a limited extent [5]. The fingertip yielded more volume and fewer failed collections, while the upper-arm device produced less haemolysis [5].
At Mayo Clinic, participants collected capillary blood with Tasso+, each paired with a venous sample (78 collections, 65 of them analysable) [6]. In this study, creatinine, blood lipids and PSA proved clinically equivalent to the venous sample, as did the thyroid values (see below). Potassium, bicarbonate and glucose were unstable [6].
For liver values, there is a study of 36 patients with abnormal liver tests: ALT, AST, ALP and bilirubin from Tasso+ samples agreed very closely with the venous draw (CCC > 0.99, centrifuged samples) [22].
Thyroid
For TSH and free T4 (fT4), several comparison studies show close agreement. In a study of 55 participants using an upper-arm device (TAP II), capillary values correlated with venous values at r = 0.99 for TSH and r = 0.97 for fT4 [9]. In a study of 130 women, the correlation between fingertip and venous samples was 0.99 to 1.0 for TSH and fT4, as it was for the fertility hormones studied [7].
In the Mayo Clinic study, TSH, fT4, T3 and TPO antibodies from Tasso+ samples were clinically equivalent to the venous sample. The thyroid values remained acceptable even after 48 hours at room temperature and at 40 °C [6].
A further study of 39 analytes reported a correlation of r = 0.991 for TSH with a bias of −0.6% [23]. The authors report equivalence with conventional collection for the routine analytes studied. In that study, however, trained staff collected the samples with a dedicated device (BD MiniDraw); it was not self-collection [23].
Hormones
In the study of 130 women on day 3 of the menstrual cycle, AMH, estradiol, FSH, LH, prolactin and testosterone from the fingertip agreed closely with venous values (correlation 0.99 to 1.0, no systematic bias) [7].
In an anti-doping setting, 20 people were tested weekly for six weeks [8]. Testosterone, the testosterone to androstenedione ratio (T/A4), LH and the testosterone to LH ratio showed excellent agreement between venous serum and Tasso+ capillary serum [8].
For anti-Müllerian hormone (AMH), the result depends on the method. With an upper-arm device, the coefficient of determination against venous blood was R² = 0.99 (41 women) [10]. In a study with fingertip collection in 90 women, the correlation was high (r = 0.982), but the values showed a proportional deviation (mean 2.74 vs. 3.42 ng/ml) [24]. The authors therefore do not yet consider capillary and venous values fully interchangeable [24].
For cortisol, a small study of 11 men before and after an exercise test showed close agreement between capillary and venous blood (R² = 0.96, no systematic bias) [25].
Two limitations apply specifically to hormones: separator gel in the sample tube, and hormone gels or creams on the skin. Both are described in the section Where the limits are.
Blood count
Several anti-doping studies have examined the blood count from Tasso+ samples (EDTA). In 58 paired samples collected under official doping control conditions and shipped chilled by air, all blood count parameters agreed very well with venous blood, provided the samples showed no visible clots [12]. The exception was platelets [12]. A comparison of Tasso+ and a second device (RedDrop) with venous blood in 10 participants and 30 samples reached the same result [13]. Haemoglobin, reticulocytes and the OFF score also showed very good agreement; the capillary samples were stable for up to 72 hours at 4 °C [14].
In a study of 93 patients, full blood count parameters from capillary and venous blood showed a strong linear correlation, and deviations were within the acceptable range [15].
Stability during shipping
In home sampling, the postal route and changing temperatures lie between collection and analysis. How stable a parameter remains under these conditions determines how meaningful the result is.
One study examined 47 routine and immunochemistry markers in whole blood at 4–8 °C, 20–25 °C and 37 °C for up to 72 hours [3]. Most markers remained stable when refrigerated and at room temperature. Potassium, inorganic phosphate and iron fell outside acceptable limits under several conditions; sodium, calcium, ferritin, AST and CYFRA 21-1 became unstable only at 37 °C [3]. An important caveat: the study used whole blood from pools and from two volunteers, not capillary samples. The authors describe their work as a first step before validation under real capillary conditions [3].
For the blood count, temperature plays a major role. At 4 °C, haemoglobin, haematocrit, MCV and platelets, among others, remained stable for at least 96 hours [15]. At 30 °C, all parameters except erythrocytes, haemoglobin, leucocytes and MCH exceeded the allowable deviation after 24 hours [15]. Another study found stable values for erythrocytes, platelets, haemoglobin and MCH over 48 hours at 4, 10 and 23 °C [16]. At 23 °C, haematocrit and MCV rose after 48 hours, leucocytes fell, and the differential blood count changed from 12 hours onwards [16]. A meta-analysis of 89 studies concluded that blood count parameters other than MPV are stable for at least 24 hours, and leucocytes, platelets, haematocrit, haemoglobin and MCH for up to three days [26]. For longer storage, the authors recommend refrigeration [26].
How home collection with postal return works in practice is shown by a study in patients with inflammatory bowel disease. Samples were in transit for less than 48 hours, with a temperature sensor monitoring a window of 4 to 30 °C [17]. At the first appointment, 81% of home samples could be analysed, at the second 95%; 57% of participants preferred home collection [17].
Where the limits are
For some parameters and under certain conditions, the good agreement does not hold. The table summarises where studies show limitations and what follows from them.
| Parameter / situation | What the studies show | Consequence |
|---|---|---|
| Potassium, bicarbonate, chloride | potassium and bicarbonate deviated to a clinically relevant extent in capillary samples, especially after delayed processing [6]; potassium and chloride only interchangeable to a limited extent [5]; potassium unstable in whole blood [3] | not suitable from capillary blood |
| Glucose | unstable with delayed processing [6]; the only one of 29 parameters that failed the CLIA criteria [4] | use HbA1c for glucose metabolism, or another sample type |
| Phosphate, iron | outside acceptable limits in whole blood under several storage conditions [3] | only with the laboratory’s own stability validation |
| Platelets | the only blood count parameter that did not agree well, regardless of device [12, 13] | not suitable from capillary blood |
| Haematocrit, MCV, differential blood count | change during storage at room temperature: haematocrit and MCV rise after 48 h at 23 °C, the differential changes from 12 h [16]; at 30 °C most blood count parameters exceeded the allowable deviation after 24 h, while erythrocytes, haemoglobin, leucocytes and MCH remained stable [15] | only meaningful with short transport times or refrigeration |
| Separator gel in the tube | separator gel in small capillary tubes measurably lowered estradiol and testosterone (by about 6 to 19%, depending on the comparison); gel-free capillary tubes gave values comparable with venous blood [11] | tube choice is part of laboratory validation |
| Hormone gels and creams | after progesterone cream or gel had been applied, capillary levels were many times (about 100-fold) higher than in venous serum [18] | when hormone gels or creams are used, capillary results are not comparable with venous results |
| AMH | in a study with fingertip collection, proportional deviation despite high correlation (mean 2.74 vs. 3.42 ng/ml); the authors do not yet consider the values fully interchangeable [24]; with an upper-arm device R² = 0.99 against venous [10] | result depends on device and method |
| Heat during transport | at 37 °C, sodium, calcium, ferritin and AST also became unstable [3] | ship without exposure to heat wherever possible |
| Sample volume | in one study, 17–22% of self-collections were insufficiently filled [6]; in a home study, 81% and 95% of samples could be analysed at the first and second appointment respectively [17]; the fingertip yielded more volume than TAP Micro Select, but more haemolysis [5] | clear instructions; repeat collection if a sample cannot be analysed |
How we assure quality
Probatix only offers a parameter from capillary blood if the analysing partner laboratory has validated it for this sample type: in a method comparison with venous samples and in stability studies under realistic shipping conditions. Anything that does not pass this testing is not included in the capillary blood portfolio. All samples are analysed in ISO 15189-accredited partner laboratories, and results are released by a physician. For collection we use vacuum-assisted upper-arm devices such as Tasso+ and TAP Micro Select; instructions and return materials are designed for them. Sample receipt and sample quality are monitored systematically.
The home sampling page shows how Probatix runs home sampling as a service, from sample types to return logistics. The regulatory difference between self-tests and home sampling is explained in the article on the IVDR, and the key terms in the glossary.