The blood is obtained from the capillaries contained within the skin. The arterial–venous network of the skin is located at the junction of the lower dermis and upper subcutaneous tissue. The skin should be punctured only to the depth of this junction to facilitate blood flow; a deeper puncture can have serious complications. If the calcaneus (heel bone) is punctured, there is a risk of osteochondritis or osteomyelitis resulting. The distance between the skin and bone can vary depending on where on the foot measurement is taken (with the narrowest distance being at the posterior curve of the heel) and the weight and gestation of the baby.

Another consideration is the position of plantar arteries and nerves, which should be avoided. Puncturing the arteries can result in haemorrhage and increases the risk of introducing infection, which could result in septicaemia. Puncturing the nerves can result in permanent damage to the nerve.

Blumenfeld et al (1979) estimated the distance from the surface of the skin to the arterial–venous network to be 0.35–1.6 mm (at postmortem). Jain & Rutter (1999) attempted to replicate this work with live babies and argue that none of the babies in their study had a distance of less than 3 mm between capillaries and skin, and thus argue that any part of the plantar surface would be suitable for pricking. However, more research is necessary and this is not yet incorporated into current practice. Automated lancets (generally) puncture to a depth of 2.4 mm or less (often 1 mm).

Blood spot screening is offered to all babies and should be taken between 5 and 8 days after birth (the day of birth being day 0), ideally on day 5. Testing at this stage is necessary to instigate treatment as early as possible. Many disorders could be found from these blood spots; however, screening programmes have to consider issues such as sensitivity and reliability, cost effectiveness and likely benefit. Hence, at this time, the UKNSC (2008) recommends screening from blood spot tests for:

The blood spot form is accompanied by its own glassine envelope. A type of blotting paper is used for the drops of blood; this allows the blood to soak through to the required depth for accurate testing. There are four circles, each one needs to have one drop of blood in it that fills the circle fully. Inaccurate results may be obtained if the blood:

Repeat tests are requested if:

One of the UKNSC’s standards (2008) is that the form should be completed fully, contemporaneously and with the baby’s NHS number (ideally a bar-coded label given to parents on discharge from hospital). Parents also need to be able to give informed consent for the test; the UKNSC recommends the leaflet ‘Newborn blood spot screening for your baby’ (downloadable), suggesting that it is given antenatally and then discussed again at least 24 hours prior to the test. In the event of declining the test ‘DECLINE’ should be written on the form, and it should be forwarded to the laboratory as for any others. Parents should receive the result before their 6- to 8-week neonatal assessment.

A capillary sample may be taken to estimate the level of unconjugated bilirubin in the blood of a jaundiced baby. In some maternity units, the sample is then sent to the pathology laboratory, whereas other maternity units have the equipment (centrifugal machine) to undertake the test themselves. Usually two thin, preheparinized capillary tubes are filled with blood. Care should be taken to avoid getting air in the capillary tubes as this may result in the blood dispersing totally from the tubes during spinning, necessitating a further blood test.