Maternal blood tests
Maternal blood may be taken for a variety of reasons, routinely or in an emergency during pregnancy or labour. Normal reference ranges for blood results may vary slightly between different hospitals or laboratories and are normally based on population studies.
The word ‘normal’ may need qualification. A blood result may fall within the ‘normal’ accepted range, but it may be abnormal for a particular woman. Equally, a value may fall outside the ‘normal’ range but the woman is quite well and simply has a ‘numerically non-standard’ blood result.
This needs consideration particularly when allowing for the changes in haemodynamics in pregnancy. The increase in circulating plasma volume is 50% above nonpregnant values by the 34th week of gestation, lowering haemoglobin, haematocrit and red cell counts as well as placing an increased demand on maternal organs. This should be remembered when evaluating normal reference ranges for the pregnant women.
Taking blood
- Explain why you want to take blood and gain the woman’s consent. She should subsequently be informed of the result and its significance: ensure she knows how and when this will be communicated to her.
- Always wear gloves for protection.
- Select a site: the antecubital fossa is usually best, as the cephalic, median, cubital and basilic veins are easily accessible near the skin surface (Coates, 1998).
- Collect blood into the correct bottles (there is no national colour coding for blood bottles at present), store appropriately with a completed request form and send to the laboratory as soon as possible. Record bloods taken in the mother’s notes.
Biochemical tests
Electrolytes
Lithium heparin blood bottles
- Sodium (Na) is indirectly related to body water volume.
- Potassium (K) is important for normal cardiac electrical activity, and very high or low concentrations are associated with cardiac electrical abnormality (such as ventricular fibrillation or asystole) (Table 23.1).
Table 23.1 Electrolytes.
| Constituent | Normal range |
| Na | 135–145 mmol/l |
| K | 3.4–5.2 mmol/l |
Renal function tests
Lithium heparin blood bottles
- Creatinine is a nitrogenous end product of muscle metabolism. Creatinine is filtered by the glomeruli in the kidney, so the renal clearance rate provides an approximate measurement of the glomerular filtration rate. As the concentration of creatinine can be readily measured in the plasma, it is a useful indicator of renal function, particularly when sequential observations are made (Walton et al., 1994).
- Uric acid is the end product of protein metabolism. Elevated uric acid levels may reflect decreased renal blood flow caused by vasoconstriction.
- Urea is a waste product of metabolism which is excreted via the kidneys (Table 23.2).
Table 23.2 Renal function tests.
| Constituent | Normal range |
| Creatinine | 60–120 mmol/l |
| Uric acid | 0.2–0.4 mmol/l |
| Urea | 2.5–6.5 mmol/l |
Liver function tests
Studies suggest that pregnancy liver enzymes are lower than the non-pregnant reference ranges often used and that, in the absence of altered hepatic blood flow, physiological haemodilution alone may result in lower values for alanine transaminase (ALT), aspartate transaminase (AST) and bilirubin levels (Girling et al., 1997) (Table 23.3).
Table 23.3 Liver function tests.
| Constituent | Normal range |
| ALT | 6–40 U/l |
| AST | 10–40 U/l |
| ALP | 40–120 U/l (↑> ++ at term so of no diagnostic value) |
| Albumin | 34 g/l |
| Bilirubin | 5–17 µmol/l |
Alanine transaminase and aspartate transaminase
Lithium heparin blood bottles
The activities of ALT and AST are widely used as a sensitive, although non-specific, index of acute damage to liver cells, irrespective of its cause (Gaw et al., 1999). Levels remain unchanged in normal pregnancy.
Alkaline phosphatase
Lithium heparin blood bottles
While other liver enzymes remain unchanged in normal pregnancy, alkaline phosphatase (ALP) does not. It is produced by the placenta from the first trimester onwards: by the third trimester it is so greatly raised that it has virtually no diagnostic value.
Total albumin
Clotted blood sample
Plasma albumin is also synthesised in the liver and so indicates liver function. In normal pregnancy the decrease in albumin levels is caused by haemodilution and not as a result of liver insufficiency (McKay, 1999).
Total bilirubin
Lithium heparin blood bottles
This test is to screen the liver for damage and to investigate the causes of anaemia. Haemoglobin destruction results in the production of bilirubin, which is conjugated in the liver and excreted in the bile. Any overload or blockage of the system raises levels (Bratt-Wyton, 1998). During a normal pregnancy levels do not usually rise. However, in HELLP (haemolysis, elevated liver enzymes, low platelets) syndrome levels can sometimes increase.
Haematological tests
Full blood count
Ethylenediamine-tetra-acetic acid (EDTA) blood bottles
- Haemoglobin (Hb) is the pigment contained in the red blood cells and enables them to transport oxygen around the body. Anaemia may cause tiredness and loss of energy: it has little effect on labour itself but can potentiate the effect of any haemorrhage at birth. There are no definitive ranges for Hb as there is a lot of variance in the literature. National Institute for Clinical Excellence (NICE) (2003) recommends iron supplementation if <10.5 g/dl after 28 weeks, but an Hb of >9 g/dl in labour is unlikely to cause major problems. Low Hb is often due to haemodilution of pregnancy; women are often falsely diagnosed as anaemic and treated unnecessarily with iron supplements, without consideration of the full blood picture. Interpret the Hb in the light of the MCV, MCH and haematocrit. If the haematocrit is low, and the MCV and MCH are normal, anaemia is unlikely.
- Platelets are essential for normal haemostasis. Platelets may be reduced in preeclampsia and HELLP syndrome. The function of platelets is related to many factors in the body’s coagulation system. A platelet count is important as thrombocytopaenia (deficiency of the platelets) frequency accompanies other disorders (Star & Peipert, 1996).
- White blood cells (WBC). This test calculates the total number of all the different white cells. It indicates bone marrow health and if the immune system is being stimulated for any reason (Frye, 1998).
- Haematocrit or packed cell volume (PCV). This shows the concentration of red cells in the plasma. As with Hb, in normal pregnancy the haematocrit level will decrease during the second trimester. If the haematocrit is low, this means the blood is dilute and the Hb is likely to be low as well, as Hb is measured relative to the plasma volume.
- Mean corpuscular volume (MCV) is the average volume of a single red cell. It has been regarded as the most sensitive red cell index for the identification of iron deficiency. Values below 70 fl occur only with iron deficiency anaemia or thalassaemia minor (Kirkpatrick & Alexander, 1996).
- Mean corpuscular haemoglobin (MCH) measures the amount and volume of haemoglobin inside the red blood cell. A woman with a low Hb and normal MCH and MCV levels will probably not be anaemic (Table 23.4).
Table 23.4 Full blood count.
| Constituent | Normal range |
| Hb | 8–14 g/dl |
| Platelets | 150–350 × 109/l |
| WBC | 6–18 × 109/l |
| PCV | 36–48% |
| MCV | 80–96 fl |
Serum ferritin
Ferritin is a protein inside the cell that stores iron for future use. Serum ferritin levels demonstrate how much iron the body has stored, and are therefore a more accurate (but more expensive) test for anaemia than haemoglobin estimation, which is affected by haemodilution. Normal levels range from 15 to 250 ng/ml.
Clotting screening
Sodium citrate blood bottles
Samples must be tested as soon as possible on the day of collection.
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