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When you have completed this skills chapter you should be able to:
The packed cell volume (PCV or haematocrit) is the percentage of red cells in a sample of whole blood. The normal packed cell volume is 45 to 65% at birth (i.e. just over half the volume of blood consists of red cells). In the newborn nursery the packed cell volume, rather than the haemoglobin concentration, is measured as it is more accurate. It is also more convenient as often the blood has to be spun in order to measure the total serum bilirubin concentration. The packed cell volume is approximately 3 times the haemoglobin concentration (Hb), i.e. a PCV of 30% would be expected if the Hb was 10 g/dl.
The packed cell volume may be measured on a sample of arterial, venous or capillary blood. If capillary blood is used, very careful attention must be paid to the correct method of collecting the sample. Usually blood is sampled from the infant’s heel. The infant’s foot must be warm so that the blood flows easily without the need to squeeze the heel. If the heel is cold and has to be squeezed, an incorrectly high-packed cell volume reading may be obtained.
The method of obtaining a capillary blood sample for a packed cell volume measurement is as follows:
Be very careful not to prick your finger by mistake when obtaining a sample of capillary blood from an infant. Immediately after piercing the infant’s skin, the lancet must be placed in a special container for ‘sharps’. It is very important to dispose of the lancet as soon as possible as it is very easy to prick yourself while removing the used equipment after the procedure. Therefore, never leave a used lancet or needle lying exposed. Viruses such as hepatitis B and HIV can be transmitted as a result of a finger prick with a lancet or needle if the patient is infected.
Place the lancet in a special container for ‘sharps’ immediately after piercing the skin.
If possible, clear plastic rather than glass capillary tubes should be used. This avoids the risk of a tube breaking in the microcentrifuge. Blood-stained broken glass is dangerous as it may cut the operator’s finger and spread HIV. It is best to always use gloves when collecting a blood sample.
A safety lancet is expensive and only used once. However, it avoids the risk of a ‘needle stick injury’.
The sample of blood in the capillary tube must be spun down in a microcentrifuge for 2 minutes. All level 2 and 3 nurseries should have a microcentrifuge.
A microcentrifuge should be used as follows:
You will notice that the red cells have all been spun to one end of the tube. The rest of the tube is filled with serum. Where the red cells and the serum meet, you will see a 1 mm white band. This is formed by the white blood cells.
A special instrument called a packed cell volume reader (or a micro-haematocrit reader) is used to measure the packed cell volume. Two different types of PCV reader are available. One type measures the PCV while the capillary tubes are still in the microcentrifuge while the other type is completely separate from the microcentrifuge.
Measuring the packed cell volume on the microcentrifuge:
Measuring the packed cell volume off the microcentrifuge:
Before the total serum bilirubin (TSB) can be measured with a bilirubinometer, a sample of blood has to be collected into a capillary tube and spun down as described above. A number of different types of bilirubinometers are available. Some measure the TSB in the serum while still in the capillary tube. Others require that the tube has to be snapped at the junction of the serum and the red cells so that the serum can be run into a special glass measuring chamber. This type is dangerous if the HIV prevalence in the community is high as it is very easy to cut one’s finger when snapping a glass tube. It is safest to use clear plastic capillary tubes with a bilirubinometer which measures the TSB in intact tubes. Both types of bilirubinometer are electrically powered, expensive and have to be carefully standardised at least once a week. The care and use of a bilirubinometer, therefore, should be the responsibility of a trained medical technologist. Staff wishing to use a bilirubinometer should get personal instructions from the local technologists. The method is simple but differs depending on the model of bilirubinometer available.
A phototherapy unit is used to provide a source of bright light to treat jaundice in newborn infants. The light alters the bilirubin in the skin of the infant allowing the infant’s liver to excrete the bilirubin which is now water soluble.
A number of commercial or hospital-made phototherapy units are available. Usually they have fluorescent tubes although some have white halogen or blue LED (light emitting diode) spot lights. The main components of a fluorescent tube phototherapy unit are:
The light box: The fluorescent tubes are fixed into a special light box which usually has a fan to keep the tubes cool. Otherwise the tubes and the infant will overheat. Most light boxes have a time counter which indicates how many hours the tubes have been used.
A thick sheet (1 cm) of clear perspex (plastic) must be placed under the tubes to protect the infant from falling glass if a tube explodes. The perspex does not lessen the effect of the phototherapy but it does reduce the amount of heat reaching the infant.
Some overhead radiant heaters (intensive care cribs) have white halogen spot lights or blue LED spotlights which can be used to give phototherapy.