Pricked-heel blood

A circle 3 mm in diameter (roughly corresponding to 3 fxL of original blood) is punched out from each spot by means of a standard hole puncher into 1.5 mL polypropylene tube. Specimen is pricked-heel blood subsequently dried on Schleicher Schuell filter paper (in Europe Grade 903 is used at the most). 

From the fingertip, earlobe or heel prick, one would like to stay within the limits of 100 - 200p 1 as a maximum. If one then draws 100 1 of blood, then one cannot hope to have more than approximately 40pi of plasma, since the hematocrit of newborn infants is relatively high. This volume would be substantially lowered, in the dehydrated infant, with which one is working, normally. 

The patient then needs to be prepared. In the case of the heel prick, a leg of the infant is massaged or warmed so as to increase circulation to the heel where the puncture is to be made. The site from which the collection of blood is made, is determined by the size of the infant. In prematures, where the fingers are extremely tiny, one has no choice but to obtain the specimen from the heel. In older children, the large toe may be used. In using the heel, one does not use the bottom of the heel because here the capillaries lie deep. One uses instead the back of the heel, where the capillaries come near the surface. Generally, sterile scapel blades or lances are used to puncture the skin and obtain the blood flow. Figure 8 illustrates the process of obtaining the specimen. 

Newborn screening tests are done on a small blood sample, which is taken by pricking the baby s heel. Unlike other types of genetic testing, a parent will usually only receive the result if it is positive. If the test result is positive, additional testing is needed to determine whether the baby has a genetic disorder. 

Although this detail of the work is only partly dependent on specialized apparatus and is described in various handbooks, it seems wise to give an outline of the possibilities of modem techniques, emphasizing some new ideas. Collecting material usually means collecting blood. For microtechniques two drops is enough for each analysis, so cumbersome venipunctures can be replaced by earlobe, heel-, or fingertip prick the differences in composition between venous and capillary blood are very small for the majority of constituents (Kl). 

Fig. 2.1 Newborn heel-prick and spotting of blood onto filter paper for newborn screening (Photo courtesy of Erica Wright, MS, CGC)...

Blood samples suitable for analysis can be drawn by syringe or vacutainer from the antecubital vein or collected in a capillary tube after pricking a finger or the heel (in the case of infants) with a lancet. Blood samples of 5-10 ml drawn by syringe from a vein are preferred to capillary samples, inasmuch as hemolysis is usually less of a problem, dilution of the blood with extracellular fluids is minimized, and analytical reproducibility is better because of the larger sample size. In either case, however, acceptable values for retinol can be obtained from blood samples as small as 0.1-0.2 ml. 

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