If venous blood from a normal man or woman is left to clot it will, as a rule, form a homogenous red mass at the end of the coagulation.
In certain pathologic and physiologic states, the coagulum is differentiated into a lower red and an upper, more or less broad, yellowish layer (crusta phlogistica, buffy coat, size), which consists of coagulated plasma.
While this phenomenon is normal in the blood of several animals, it has been known from the earliest times that a crusta phlogistica in human blood is a pathologic phenomenon.
The newer medicine, however, generally has disregarded this symptom, possibly because it may be found in so many different states, which did not seem to form a unity. This opinion, for instance, is thus expressed by Panum1 (1851): "Utrum crusta in sanguine appareat, necne, ex rationibus compluribus vere physicis pendet, et vix unquam licet inde certam sanguinis compositionem
GRAM HC. ON THE CAUSES OF THE VARIATIONS IN THE SEDIMENTATION OF THE CORPUSCLES AND THE FORMATION OF THE CRUSTA PHLOGISTICA ("SIZE," "BUFFY COAT") ON THE BLOODA PRELIMINARY COMMUNICATION. Arch Intern Med (Chic). 1921;28(3):312–330. doi:10.1001/archinte.1921.00100150077005