Background
A sensitive and specific method of detecting the presence of CSF is based on the different distribution of isoforms of transferrin, a protein involved in ferrous ion transport.
- Beta-1-transferrin can be found in serum, nasal secretions, tears and saliva
- Beta-2-transferrin is an asialated form and is confined to CSF (15% of total transferrin), perilymph and acqueoushumor
The two isoforms have structural differences which result in differences in electrophoretic mobility. An electrophoretogram of the fluid in question is run, followed by immunofixation with antisera to transferrin. In normal serum, only one transferrin band is detected whilst normal nasal secretions have no bands unless contaminated with serum. By contrast, two distinct bands are found in CSF and CSF contaminated fluids, since beta-2-transferrin migrates more slowly towards the positive electrode (cathode).
An additional clue to the presence of CSF is detection of a prealbumin band on EPG of the test fluid. Very rarely, a false positive result may be due to alcoholic cirrhosis or genetic transferrin variants. In the case of alcohol exposure, the transferrin isoform loses 2 sialic acid residues and migrates between beta-1 and beta-2 bands. Analysis of concommitant serum easily identifies the presence of a second band and is suggested if excessive alcohol intake is suspected, not uncommon in the setting of significant head injury.
Sample Requirement
- 0.5ml body fluid
- Plain tube, no additive
- Transport at room temperature ASAP
- If delayed more than 2 hrs, send at 4°C
- An additional 1ml serum is recommended if excessive intake of alcohol is suspected