Erythropoietin(EPO)isaheavilyglycosylatedproteinwithamolecularweightofabout30.000–34.000Daltons.HumanEPOisapolypeptideconsistingof165aminoacids,containingoneO-linkedandthreeN-linkedcarbohydratechains1.TherecombinantEPOisagoodsubstituteforthenativeproteinforuseinanimmunoassay2.SerumEPOlevelsaredependentontherateofproductionandtherateofclearanceoftheprotein.NinetypercentofEPOisproducedintheperitubularcellsoftheadultkidneyinresponsetoadecreaseintissueoxygenation3,4.Thereisevidenceindicatingthattheproteinonthesecellswhichdetectsoxygensaturationofthebloodisaheme-containingmoiety5.AsthepO2oftheplasma,afunctionofthehematocritdecreases,EPOconcentrationwillincrease6.TherearealsoobservationssuggestingthatnormallythereisaninversecorrelationbetweenserumEPOlevelsandredbloodcellmass7.Quantitationofserumerythropoietinconcentrationservesasadiagnosticadjunctindeterminingthecauseofanemiaorerythrocytosis.Aplasticanemia,hemolyticanemiaandanemiaduetoirondeficiencyallresultinserumEPOelevation.Whereas,EPOlevelsinpatientswithsecondaryanemiaduetorenalfailureandotherdisorderssuchasacquiredimmunedeficiencysyndrome(AIDS)aregenerallyinappropriatelylowforthedegreeofanemia.ThisismostlylikelycausedbyanimpairedABIlityofthediseasedkidneytoproduceadequatequantitiesofEPO8.LowconcentrationsofEPOmaygiveanearlywarningofkidneytransplantrejection10.EPOalsocanbeusedtomonitorAIDSpatientsundergoingZidovudine(AZT)therapy.AnincreasedconcentrationofEPOverifiesthatanemiaassociatedwithAZTtherapyisduetoredcellhypoplasiaorapliasia10.Polycythemiarubravera,orprimaryerythrocytosis(anincreaseofredbloodcellmass)resultsfromunstimulatedoverproductionoferythrocytes.Hence,theincreaseinthehemoglobincausesdecreasedproductionofEPO,whichresultsinsubnormallevelsofserumEPO9.Secondarypolycythemias,whicharealsocharacterizedbyanincreaseinthetotalredbloodcellmass,occurasaphysiologicalresponsetoelevatedlevelsofcirculatoryEPOcausedbytissuehypoxia.Thehypoxiamaybeduetosuchfactorsaspulmonaryfibrosis,cardiovasculardisease,prolongedexposuretohighaltitude,abnormalformsofhemoglobinordrugtreatment10.SometumorsproduceEPOand,inthesecases,EPOmaybeusedasatumorMarkertomonitortheeffectivenessoftreatment.