Which tubes will not be collected by dermal puncture
The information given here supplements that given in Chapter 2. Users of these guidelines should read Chapter 2 before reading the information given below. This chapter covers background information (Section 7.1), practical guidance (Section 7.2) and illustrations (Section 7.3) relevant to capillary sampling. Show
Capillary sampling from a finger, heel or (rarely) an ear lobe may be performed on patients of any age, for specific tests that require small quantities of blood. However, because the procedure is commonly used in paediatric patients, Sections 7.1.1 and 7.1.2 focus particularly on paediatric capillary sampling. 7.1. Background information on capillary sampling7.1.1. Choice of siteAdult patientsThe finger is usually the preferred site for capillary testing in an adult patient. The sides of the heel are only used in paediatric and neonatal patients. Ear lobes are sometimes used in mass screening or research studies. Paediatric and neonatal patientsSelection of a site for capillary sampling in a paediatric patient is usually based on the age and weight of the patient. If the child is walking, the child's feet may have calluses that hinder adequate blood flow. Table 7.1 shows the conditions influencing the choice of heel or finger-prick. Table 7.1Conditions influencing the choice of heel or finger-prick. Specimens requiring a skin puncture are best obtained after ensuring that a baby is warm, as discussed in Section 6.2.2. 7.1.2. Selecting the length of lancetAdult patientsA lancet slightly shorter than the estimated depth needed should be used because the pressure compresses the skin; thus, the puncture depth will be slightly deeper than the lancet length. In one study of 52 subjects, pain increased with penetration depth, and thicker lancets were slightly more painful than thin ones (67). However, blood volumes increased with the lancet penetration and depth. Lengths vary by manufacturer (from 0.85 mm for neonates up to 2.2 mm). In a finger-prick, the depth should not go beyond 2.4 mm, so a 2.2 mm lancet is the longest length typically used. Paediatric and neonatal patientsIn heel-pricks, the depth should not go beyond 2.4 mm. For premature neonates, a 0.85 mm lancet is available. The distance for a 7 pound (3 kg) baby from outer skin surface to bone is:
The recommended depth for a finger-prick is:
Too much compression should be avoided, because this may cause a deeper puncture than is needed to get good flow. 7.1.3. Order of drawWith skin punctures, the haematology specimen is collected first, followed by the chemistry and blood bank specimens. This order of drawing is essential to minimize the effects of platelet clumping. The order used for skin punctures is the reverse of that used for venepuncture collection. If more than two specimens are needed, venepuncture may provide more accurate laboratory results. 7.1.4. ComplicationsComplications that can arise in capillary sampling include:
7.2. Practical guidance on capillary sampling7.2.1. Selection of site and lancet
7.2.2. Procedure for capillary samplingAdult patientsPrepare the skin
Take laboratory samples in the correct order to minimize erroneous test results
Paediatric and neonatal patientsImmobilize the child
Prepare the skin
Puncture the skin
Take laboratory samples in the order that prevent cross-contamination of sample tube additives
Give follow-up careThere are two separate steps to patient follow-up care – data entry (i.e. completion of requisitions), and provision of comfort and reassurance. Data entry or completion of requisitions
Comfort and reassuranceShow the child that you care either verbally or physically. A simple gesture is all it takes to leave the child on a positive note; for example, give verbal praise, a handshake, a fun sticker or a simple pat on the back. A small amount of sucrose (0.012–0.12 g) is safe and effective as an analgesic for newborns undergoing venepuncture or capillary heel-pricks (70). Unsuccessful attempts in paediatric patientsAdhere strictly to a limit on the number of times a paediatric patient may be stuck. If no satisfactory sample has been collected after two attempts, seek a second opinion to decide whether to make a further attempt, or cancel the tests. 7.3. Illustrations for capillary samplingFigure 7.1Capillary samplingWhich test can not be collected by dermal puncture?Capillary punctures are not suitable for blood culture testing and most coagulation tests.
What tube is used for dermal samples?This standard recommends that EDTA tubes be drawn first to ensure good quality specimen, followed by other additive tubes and finally, serum specimen tubes.
What test can be collected using a dermal puncture?Phlebotomists and point-of-care instrument operators (laboratory, nursing and respiratory therapy personnel) frequently need to perform dermal punctures to collect capillary blood specimens for testing.
What areas should be avoided in selecting a dermal puncture site?Avoid a previous puncture site. Punctures must never be performed on the posterior curvature of the heel, the arch of the foot, or on fingers of a newborn or infant less than one year old.
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