Blood sampling and blood film preparation and examination

Abstract

Performing an accurate blood count and correctly interpreting a blood film require that an appropriate sample from the patient, mixed with the correct amount of a suitable anticoagulant, is delivered to the laboratory without undue delay. No artefacts should be introduced during these procedures. The identity of the patient requiring blood sampling should be carefully checked before performing a venepuncture. This is usually done by requesting the patient to state surname, given name and date of birth and, for hospital inpatients, by checking a wristband to verify these details and, in addition, the hospital number. To reduce the chance of human error, bottles should not be labelled in advance. The person performing the phlebotomy must conform to local guidelines, including those for patient identification. Although traditionally more attention has been given to patient identification in relation to blood transfusion, it should be noted that wrong treatment has also followed the misidentification of patients from whom samples are taken for a blood count and identification must also be taken seriously in this field. More secure identification of inpatients can be achieved by the use of electronic devices in which the patient’s identity is scanned in from a bar‐ coded wristband by means of a hand‐held device. Patients should either sit or lie comfortably and should be reassured that the procedure causes only minimal discomfort; they should not be told that venepuncture is painless, since this is not so. It is preferable for apprehensive patients to lie down. Chairs used for venepuncture should preferably have adjustable armrests so that the arm can be carefully positioned. Armrests also help to ensure patient safety, since they make it harder for a fainting patient to fall from the chair. I have personally observed one patient who sustained a skull fracture when he fainted at the end of a venepuncture and fell forward onto a hard floor, and two other patients, neither previously known to be epileptic, who suffered epileptiform convulsions during venepuncture. Such seizures may not be true epilepsy, but consequent on hypoxia following brief vagal‐induced cessation of heart beat [1]. If venepunctures are being performed on children or on patients unable to cooperate fully, then the arm for venepuncture should be gently but firmly immobilized by an assistant. Gloves should be worn during venepuncture, for the protection of the person carrying out the procedure. Non‐latex gloves must be available if either the phlebotomist or the patient is allergic to latex. The needle to enter the patient must not be touched, so that it remains sterile. In some circumstances, the patient should rest prior to venesection. In endurance athletes being tested for a ‘biological passport’, 10 minutes’ rest in a seated position has been found to be sufficient for the haemoglobin concentration (Hb) and haematocrit (Hct) to fall to a stable level [2].