CSF pigments (absorption maxima):
Oxyhaemoglobin: 416, 540, 578 nm.
Bilirubin: either a broad peak in the range of 450-460 or a shoulder adjacent to oxyhaemoglobin.
Methaemoglobin: absorption maxima 412, 540, 578nm. The latter two peaks have characteristically different shapes from the corresponding peaks of oxyhaemoglobin.
Calculations of bilirubin: Draw predicted baseline from about 360nm to a point where this will form a tangent to the scan between 440-530nm and then measure the absorbance of the scan above this baseline at 476nm to yield net bilirubin absorbance (see typical examples referred to the section on reporting results).
The concentration of CSF bilirubin (if present) is given by:-
(A475 – A at baseline drawn above) x 23 = (bilirubin) µmol per litre
Net bilirubin absorbance greater than 0.007 is considered abnormal and therefore should be calculated using the above formula. In case of oxyhaemoglobin, absorbance values greater than 0.02 is considered reportable.