Refractometric Determination of Serum Protein Concentration. II. Comparison with Biuret and Kjeldahl Determinations

The refractometric determination of the total protein concentration in some animal plasmas

“…Babul and Stellwagen (1969) CDnsidered that variation in RI was due to differences in amino-acid compositiDn, whereas Line.s and Raine (1970a) initially considered that the method was subject not Dnly to' slightly different specific refractiDns of individual proteins, but tD the presence Df variable amounts of nDnprotein material. The latter aspect, however, was found to be unimportant because, although the proteins contributed about 85% of the total dissolved solids in serum, their contribution to the RI was considerably higher (Lines and Raine, 1970a).…”

“…For example, the Kjeldahl method, which has been regarded as a standard, depends on the determination of nitrogen content and multiplying either by a factor of 6.25 Henry, 1964) or, as suggested mDre recently, by a factor Df 6.54 (Sunderman, 1966). The: Biuret method depends on the number Df peptide bonds present (Lines and Raine, 1970b), but 'Opinion differs as to what factors are important in refracto~ metry. Babul and Stellwagen (1969) CDnsidered that variation in RI was due to differences in amino-acid compositiDn, whereas Line.s and Raine (1970a) initially considered that the method was subject not Dnly to' slightly different specific refractiDns of individual proteins, but tD the presence Df variable amounts of nDnprotein material.…”

“…It must be pDinted out that all these fDrmulae:. were derived from serum and that one Df them (Lines and Raine, 1970a) used a scale reading which cDnverted to RI by reference to 'a table supplied by the manufacturer of the instrument cDncerned. In addition, the Biuret method was not the usual reference used in the human studies, although in Dne instance, when Biuret and K i eldahl methods were compared manually, there was no significant difference between them in either accuracy or precision (Lines and Raine, 1970b).…”

“…Total protein values were read from this curve using the plasma RI values. In addition, total protein levels were calculated from the RI with formulae derived by other workers on human sera (Lines and Raine, 1970a;Drickman and McKeon, 1962;Sunderman, 1966), and by reference to a direct reading protein scale, normally ,found on protein refractometers and illustr:ated in Schalm (1965).…”

“…REFR~CTOMETRY has been used for serum or plasma total protein determination since the beginning of this century and while at times the value and accuracy of the: technique has been open to question (see Lines and Raine, 1970a,b), many studies have shawn close agreement between refractometric determination of serum Itotal protein and estimation by such techniques as the Biuret method and the Kjeldahl nitrogen de:termination (Lines and Raine, 1970b;Barry et al, 1960;Drickman and McKeon, 1962).…”