Point-of-Care Haemoglobin

Point-of-care (POC) haemoglobin testing estimates haemoglobin concentration in a drop of capillary or venous blood within 1 minute, using portable photometers (azide methaemoglobin reaction on a microcuvette, read at 570 and 880 nm), digital haematology analysers, blood gas analysers (calculated from haematocrit) and non-invasive spectroscopic devices (pulse co-oximetry).

Photometric devices have accuracy within 0.5 to 1.0 g/dL of laboratory values, are the usual field method for anaemia surveys and donor screening, and the colour scale or Sahli methods are no longer accepted. Non-invasive devices are convenient but less precise (limits of agreement up to plus or minus 2 g/dL), and are used for screening, not decisions on transfusion.

Anaemia affects about 1.9 billion people (nearly a quarter of the world), including 40% of children under 5 years and 30 to 40% of pregnant women in low income countries, and is the leading cause of years lived with disability among women.

Iron deficiency is the commonest cause, followed by infection (malaria, hookworm, HIV, tuberculosis), haemoglobinopathies, vitamin B12 and folate deficiency, chronic disease and blood loss. Rapid haemoglobin supports transfusion decisions in haemorrhage, antenatal care, malaria, sickle cell crisis, surgery and neonatal practice.

Indications. Pallor, fatigue, breathlessness, tachycardia, heart failure or syncope. Antenatal booking and each trimester. Postpartum haemorrhage and obstetric emergencies. Severe malaria and malnutrition. Trauma, gastrointestinal bleeding and surgery. Blood donor screening. Child health clinics and school surveys.

Monitoring in chronic kidney disease, cancer treatment, sickle cell disease, and iron or erythropoietin therapy. Pre-operative assessment in remote settings. Not for diagnosis of the cause of anaemia.

Specimen. Capillary blood from the side of a fingertip, from the ring or middle finger (the second drop after wiping away the first, without squeezing, which dilutes with tissue fluid), heel in infants, or venous or arterial whole blood in EDTA or heparin.

Fill the microcuvette in one continuous motion without air bubbles, wipe the outside, and read within 40 seconds to 1 minute (the instrument stated time). Cuvettes must be stored sealed and dry, because moisture alters the reagent. Record altitude, smoking, and pregnancy for interpretation.

Reference values (sea level). Adult men 13.0 to 17.0 g/dL. Adult non-pregnant women 12.0 to 15.5 g/dL. Pregnancy: first trimester 11.0 g/dL or more, second trimester 10.5 g/dL or more, third trimester 11.0 g/dL or more. Children 6 to 59 months 11.0 g/dL or more, 5 to 11 years 11.5 g/dL or more, 12 to 14 years 12.0 g/dL or more. Neonates at birth 14.0 to 20.0 g/dL.

Cut offs are raised for altitude and smoking (for example, plus 0.2 g/dL at 1000 m, plus 0.5 g/dL at 1500 m, plus 1.0 g/dL at 2000 m, and plus 0.3 g/dL for smokers).

Interpretation.

• Mild anaemia: men 11.0 to 12.9 g/dL, non-pregnant women 11.0 to 11.9 g/dL, pregnant women 10.0 to 10.9 g/dL. Moderate: 8.0 to 10.9 (7.0 to 9.9 in pregnancy). Severe: under 8.0 g/dL (under 7.0 g/dL in pregnancy and children).
• Haemoglobin under 7 g/dL is the usual trigger for red cell transfusion in stable adults (8 g/dL in cardiovascular disease and perioperative orthopaedic patients), balanced against symptoms, bleeding rate and haemodynamic state.
• In acute haemorrhage, haemoglobin is normal at first and falls as fluid shifts and resuscitation dilute the blood, so serial testing and clinical state matter more than a single value.
• Haemoglobin over 16.5 g/dL in women or 18.5 g/dL in men (or haematocrit over 49 or 52%): polycythaemia (smoking, altitude, hypoxia, dehydration, testosterone, erythropoietin secreting tumour, polycythaemia vera).
• Normal haemoglobin with symptoms does not exclude iron deficiency (ferritin is required), and iron deficient erythropoiesis occurs before anaemia.
• Anaemia with fever in an endemic region requires a malaria test, and with jaundice or dark urine suggests haemolysis.
• Haemoglobin plateau at 6 to 9 g/dL in sickle cell disease is baseline, and a fall of 2 g/dL or more suggests aplastic or splenic sequestration crisis.
• Neonatal haemoglobin under 13 g/dL in the first week suggests haemolytic disease or blood loss.

Critical values and findings needing immediate action.

• Haemoglobin under 5 g/dL, or under 7 g/dL with heart failure, shock, acidotic breathing (children), or cerebral signs: urgent transfusion, oxygen, and treatment of cause (malaria, bleeding).
• Severe anaemia in pregnancy (under 7 g/dL) near term or in labour: transfusion planning, blood cross-match, and active management of the third stage.
• Rapid fall in haemoglobin with haemodynamic instability: activation of the major haemorrhage plan and source control.
• Haemoglobin under 7 g/dL in sickle cell disease with reticulocytopenia or splenomegaly: sequestration or aplastic crisis, transfusion with caution to avoid hyperviscosity.
• Haemoglobin over 20 g/dL in a newborn (haematocrit over 65%): symptomatic polycythaemia, and partial exchange transfusion as the picture requires.
• Positive donor screen failure: haemoglobin under 12.5 g/dL (donor threshold), deferral and review.

Interferences and pitfalls.

• Capillary sampling errors: squeezing, taking the first drop, contamination with alcohol or tissue fluid, air bubbles in the cuvette, and cold or oedematous fingers. They cause underestimates of 1 to 2 g/dL.
• Lipaemia, high white cell count (over 50 times 10 to the power 9/L), high bilirubin, carboxyhaemoglobin and methaemoglobin, and some haemoglobin variants affect photometric methods. Azide methaemoglobin methods read total haemoglobin including carboxyhaemoglobin.
• Cuvettes exposed to humidity, past expiry or outside the temperature range read falsely low.
• Haemoglobin concentration depends on plasma volume, so pregnancy, overhydration, heart failure and splenomegaly dilute it, and dehydration concentrates it.
• Haemoglobin does not show the cause. Mean cell volume, reticulocytes, blood film, ferritin, vitamin B12, folate, haemoglobin electrophoresis and malaria tests are needed.
• Blood gas analysers estimate haemoglobin from haematocrit with less accuracy in extremes, and non-invasive devices (SpHb) overestimate low values and are unreliable in poor perfusion.
• Quality assurance (daily check with control cuvette or solution, staff competency, instrument calibration) is required in clinics and programmes.
• Altitude, smoking and ethnicity specific differences can change thresholds, and some groups have lower normal values.

Special groups.

• Pregnancy: dilutional fall in the second trimester; oral iron for mild anaemia and intravenous iron for severe anaemia or late presentation.
• Children: lower normal values with age, rapid growth, iron deficiency and malaria. School and preschool programmes give iron and deworming.
• Women with heavy menstrual bleeding: iron deficiency with normal haemoglobin early.
• Older adults: anaemia is never normal ageing and needs investigation for bleeding, myelodysplasia, renal disease or deficiency.
• Sickle cell and thalassaemia: baseline low haemoglobin and different transfusion thresholds.
• Regions: anaemia burden highest in South Asia and sub-Saharan Africa, with malaria, hookworm, schistosomiasis, HIV, haemoglobinopathies and nutritional deficiency contributing.

Follow up tests. Full blood count with indices, reticulocyte count, blood film, ferritin, transferrin saturation, vitamin B12 and folate, haemoglobin electrophoresis or HPLC, malaria film or rapid test, stool for occult blood and parasites, bilirubin, lactate dehydrogenase, haptoglobin and direct antiglobulin test for haemolysis, creatinine and eGFR, HIV test, bone marrow if indicated, and blood group with cross-match before transfusion.