Introduction
In neonatology, biochemical tests are used for screening, diagnosis and monitoring the effect of treatment, and the laboratory may already have been involved in antenatal testing of maternal and fetal samples.
Successful working relationships between neonatologists and the staff of clinical biochemistry departments require a mutual understanding of clinical and laboratory problems, which is often best achieved by the nomination of a designated clinical biochemist with a responsibility for the neonatal unit. Some laboratory tests considered ‘routine’ in hospitals with specialist neonatal units may be unusual, challenging or unavailable in other settings.
Centralisation of laboratory services features on some management agendas. Any changes should be planned with involvement of users taking into account all aspects of the laboratory service, not just the analytical component, so that there is no detrimental effect on service delivery.
Specimen collection
Before requesting a biochemical investigation, the neonatologist should define the question to which the analysis will help provide an answer. Various specimens can be used for biochemical tests ( Box 42.1 ), although the majority of requests are for analysis of blood. The laboratory must be provided with the correct specimen and sufficient clinical information to ensure that the correct tests can be performed and that results will be interpreted appropriately and returned correctly.
Blood
Capillary
Venous
Arterial
Urine
Faeces
Cerebrospinal fluid
Saliva
Other fluid
Synovial
Pleural
Ascitic
Calculi
Biopsy specimens
Skin
Liver
Muscle
Blood
It is essential to ensure that the correct sample is taken for the test required; this may be plasma, serum or whole blood. Plasma refers to the aqueous phase of the blood in vivo and to the supernatant obtained after centrifugation of a blood sample collected into a tube containing an anticoagulant, most commonly lithium heparin. Blood collected into a plain tube clots, and, after centrifugation, serum is obtained. Certain tests require plasma samples (e.g. ammonia), but the majority can be performed on either plasma or serum. There may be differences between the results obtained from plasma and serum samples; for example, the potassium concentration is higher in serum owing to release from platelets during clotting. If results are required with particular urgency, it is preferable to send a plasma sample, as it can be centrifuged and analysed immediately, without having to wait for the blood to clot in order to obtain the serum. Some analyses require whole blood, that is, blood collected into anticoagulant but not centrifuged, for example genetic studies and investigations of white or red cell enzymes.
Blood samples from neonates are often obtained by heel puncture (‘capillary samples’), venepuncture or from indwelling catheters, usually sited in arteries. Capillary samples contain blood from arterioles, venules and capillaries together with interstitial and intracellular fluids and are not appropriate for all tests – for example, contamination from sweat can cause spurious elevation of ammonia concentration and they should not be used to provide meaningful estimates of arterial P ao 2 . Venepuncture has been assessed to be less painful than skin puncture sampling ( ). To prevent contamination from infusate prior to taking blood from an indwelling catheter, ideally three to five times the dead-space volume should be withdrawn prior to sampling. This may not be practical in the very tiny baby. This volume should be reinfused afterwards, preferably into a peripheral intravenous line.
Sampling errors
A number of preanalytical variables can have an effect on the integrity of the sample and affect the quality of testing. These include the way in which phlebotomy is performed, the nature of the specimen tube and the way in which the sample is transported to the laboratory. Errors in standard procedure may prevent the laboratory from producing meaningful results; examples of those arising most often are shown in Table 42.1 .
| ERROR | MECHANISM | RESULT |
|---|---|---|
| Blood sampling site | Dilution of sample by intravenous fluid | Variable depending on intravenous fluid |
| Blood sampling technique | Difficulty in obtaining sample leads to haemolysis, resulting in release of red cells | ↑ Potassium |
| ↑ Phosphate and AST | ||
| Components | ||
| Stasis during venepuncture | Diffusion of plasma water from intravascular compartment and concentration of plasma | ↑ Protein, albumin and protein-bound components, e.g. calcium |
| Incorrect specimen container | Glycolysis will not be inhibited unless a fluoride/oxalate preservative is used | ↓ Glucose |
| Effect of even a slight amount of potassium EDTA (the anticoagulant in a ‘full blood count bottle’) | ↑ Potassium | |
| ↓ Calcium, ↓ ALP | ||
| ↓ Magnesium | ||
| Delay in transit to laboratory | Metabolism of glucose | ↓ Glucose |
| Leakage of cellular components | ↑ Potassium, ↑ Phosphate |
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