Haemolysis Index – Definition and Significance
The haemolysis index indicates the degree to which a blood sample is affected by the breakdown of red blood cells. It impacts the reliability of many laboratory test results.
Things worth knowing about "Haemolysis Index"
The haemolysis index indicates the degree to which a blood sample is affected by the breakdown of red blood cells. It impacts the reliability of many laboratory test results.
What Is the Haemolysis Index?
The haemolysis index is a measurement used in clinical laboratory diagnostics. It describes the extent of haemolysis in a blood sample – that is, the degree to which red blood cells (erythrocytes) have been destroyed and their contents, particularly the red blood protein haemoglobin, have been released into the blood serum or plasma. A haemolysed sample typically appears reddish or pinkish in colour.
Laboratories routinely determine the haemolysis index as part of sample quality control. Together with the lipaemia index (turbidity caused by fats) and the icterus index (yellow discolouration caused by bilirubin), it is one of the three key interference indices in laboratory medicine.
How Is the Haemolysis Index Measured?
Measurement is performed photometrically – a device measures how much light of a specific wavelength passes through the sample. Released haemoglobin absorbs light at characteristic wavelengths (around 410–415 nm and around 540 nm). The analyser automatically calculates the haemolysis index from this absorption value.
Results are commonly expressed in units such as mg/dL of free haemoglobin or as a dimensionless index value. The exact threshold values may vary depending on the analytical system and the individual laboratory.
Causes of Elevated Haemolysis in a Blood Sample
Haemolysis in a laboratory sample can have various causes:
- Pre-analytical causes (before analysis): improper blood collection, excessive shaking of the sample, incorrect storage temperature, prolonged standing before processing, use of very thin needles, or excessive suction force with a syringe
- In vivo haemolysis (within the patient): conditions such as haemolytic anaemia, transfusion reactions, severe burns, mechanical heart valves, or certain infections can cause destruction of red blood cells inside the body
- Transport-related haemolysis: vibration or temperature fluctuations during transport of the sample to the laboratory
Clinical Significance and Impact on Laboratory Values
An elevated haemolysis index is clinically significant because released haemoglobin and other intracellular components of erythrocytes (e.g., potassium, lactate dehydrogenase, aspartate aminotransferase) can distort the results of numerous laboratory parameters.
Typical laboratory values affected by haemolysis include:
- Potassium: Greatly elevated by haemolysis (pseudohyperkalaemia), as erythrocytes contain large amounts of potassium
- Lactate dehydrogenase (LDH): Falsely elevated
- Aspartate aminotransferase (AST/GOT): Elevated due to release from erythrocytes
- Haemoglobin and haematocrit: Can be distorted
- Bilirubin: May be interfered with by the colour of haemoglobin
- Troponin and other cardiac markers: Measurement may be impaired
Laboratories define individual threshold values of the haemolysis index for each parameter, above which a test result is considered unreliable or is flagged with an appropriate comment.
Handling Haemolysed Samples in the Laboratory
When the haemolysis index of a sample exceeds the defined threshold values, the laboratory has several options:
- Rejection of the sample and a request for a repeat blood draw
- Releasing the result with a warning about its limited interpretability
- Mathematical correction of the measured value, where validated correction algorithms are available
In practice, many samples are first assessed to determine whether the haemolysis is pre-analytical (caused by an error during collection or processing) or in vivo (caused by an actual medical condition). This distinction is clinically relevant, as in vivo haemolysis can itself be an important diagnostic finding.
Relevance for Patients
For patients, an elevated haemolysis index usually has no direct health implications – it is typically an indication of a technical impairment of the blood sample. In such cases, a repeat blood draw may be necessary. Only when in vivo haemolysis is present – meaning red blood cells are being destroyed within the body – is this a medically significant finding that requires further investigation.
References
- Lippi G, Plebani M, Di Somma S, Cervellin G. Hemolyzed specimens: a major challenge for emergency departments and clinical laboratories. Critical Reviews in Clinical Laboratory Sciences, 2011; 48(3): 143–153.
- Simundic AM, Bölenius K, Cadamuro J et al. Joint EFLM-COLABIOCLI Recommendation for venous blood sampling. Clinical Chemistry and Laboratory Medicine, 2018; 56(12): 2015–2038.
- Guder WG, Narayanan S, Wisser H, Zawta B. Samples: From the Patient to the Laboratory. 4th ed. Wiley-VCH, 2009.
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