Tubes containing gel (SST and PST) are NOT acceptable
This test may be vetted by a pathologist.
The clinical information for the test must be clearly written on the request form. If clinical information is not provided, or does not provide sufficient justification for the test, the test may be declined.
Declined tests :
If a test is declined, the specimen will be held for a reasonable period (usually 3 weeks but dependant on the stability of the sample). Medical practitioners seeking approval for a declined test should email the on-call Chemical Pathologist ( chemicalpathologist@adhb.govt.nz ) , giving the patient's name and NHI number and the clinical justification for the test. If unable to email, call the on-call Chemical Pathologist via Lablink (09-3078995) and identify yourself as a doctor.
Therapeutic range: Steady state: 0.6 - 3.0 mg/L
Uncertainty of Measurement: 18%
Principle : High performance liquid chromatography
The most appropriate test for patients taking warfarin is an INR. A warfarin drug level is far less useful and should not be routinely used to guide therapy. A level can be useful where there is an unexpected INR finding to distinguish vitamin K antagonism, altered protein binding, abnormal clearance or poor compliance. Occasionally, a warfarin level may be sought for accidental or deliberate poisoning. Our method does not detect other anticoagulants such as rodenticides.
The warfarin result may not accurately reflect warfarin tissue action for a number of reasons:
- Th e method measures the total (free plus bound) warfarin. The free fraction is only about 1% and more accurately represents the active (tissue) drug levels. The total level must not be over-interpreted because the free fraction can be disturbed (usually by competitive drug binding) without affecting the total warfarin.
- It does not distinguish between the warfarin R and S enantiomers , whereas S-warfarin is 3-5 times more potent than R-warfarin . These different enantiomers also have different pharmacodynamic and pharmacokinetic properties. .
- Total warfarin levels do not account for stereo-selective CYP2C9 clearance to the biologically active -hydroxywarfarin from R-isomer , or less active 6 Although having some biological activity, these hydroxylated metabolites are not measured.
- Additionally warfarin levels alone do not account for the influence of other genetic variants affecting warfarin response, e.g. VKORC1, CYP4F2 or
Ref: Lane S, Al-Zubledi S, Hatch E, et al. The population pharmacokinetics of R- and S-warfarin: effect of genetic and clinical factors. Br. J. Clin. Pharmacol. 2012; 73:66-76
Emails to chemicalpathologist@adhb.govt.nz will receive priority attention from the on-call chemical pathologist.
If the query concerns a specific patient please include the NHI number in your email.
If email is not a suitable option, please contact the on-call chemical pathologist via Lablink (Auckland City Hospital ext. 22000 or 09-3078995).
Individual chemical pathologists may be contacted but will not be available at all times.
After-hours : contact Lablink (Auckland City Hospital ext. 22000 or 09-3078995) or hospital operator for on duty staff after hours.
Dr Samarina Musaad (Clinical Lead) : SamarinaM@adhb.govt.nz ext. 22402
Dr Cam Kyle: CampbellK@adhb.govt.nz ext 22052
Dr Weldon Chiu: WeldonC@adhb.govt.nz ext. 23427
Dr Campbell Heron: CHeron@adhb.govt.nz ext. 23427
Dr Sakunthala Jayasinghe: Sakunthala@adhb.govt.nz ext. 23427
Please centrifuge as soon as possible after collection and freeze plasma at -20 o C.
Aliquot : 2mL plasma (1 mL minimum).
Send to LabPlus frozen.