Anaemia is a decrease in the total amount of haemoglobin in blood, which, except for acute blood losses, is characterised by a reduced level of haemoglobin per a unit of volume of blood. In most cases, anaemia also features a lower level of erythrocytes. However, in case of iron deficiency, thalassemia and abnormal porphyrin synthesis, the content of erythrocytes may remain normal (as to thalassemia, it may even be higher than normal), with a low level of haemoglobin. In case acute blood losses (massive haemorrhage or acute haemolysis), the level of haemoglobin and erythrocytes may remain normal during the first hours, although some other obvious symptoms of anaemia like pale integuments, conjunctivas, collapsed veins, palpitation and dyspnoea during physical activity as well as a fall in the arterial pressure (in some serious cases) appear. Anaemia is always secondary, i.e. it can be regarded as one of the symptoms of some systemic disease. Alongside frequent and easily diagnosed forms of anaemia, there are also rather rare anaemic syndromes that requires complex techniques to diagnose.