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Monitoring Low-Molecular-Weight Heparin

(Download  Monitoring Low-Molecular_Weight Heparin.pdf)

Low-molecular-weight heparin (LMWH) is increasingly being used to anticoagulate patients with venous thrombo-embolism (VTE). There is good evidence [1,2] that this is the appropriate form of anticoagulation in many patients with VTE, because of it’s ease of administration, and lack of need for monitoring, allowing patients to be treated as outpatients for all or part of their treatment. There is also some evidence of reduction in side-effects [1,2], eg heparin induced thrombocytopenia thrombosis syndrome (HITTS), and possibly bleeding.

A recent review [2] of LMWH recommended monitoring of patients on full dose LMWH who had renal failure, or whose weight was outside the range 50-80 Kg. A review of the literature [3,4,5] does not support any criteria as there is little data, some of which is contradictory. However, there are now many papers describing the use of LMWH in VTE where there is no discussion of any adjustment of dose apart from a fixed dose per Kg. Neither do any of these papers use monitoring.

There is evidence that children achieve the same anti-Xa activity as adults when administered LMWH at the same dose rate (units or mg per Kg) [6].

Whom To Monitor

After reviewing the literature, my recommendations are that adult patients only need to be monitored if they are:

  1. On full dose LMWH ie Clexane 1 mg/Kg bd SCI

PLUS one of following is evident:

  1. Serum Creatinine >400 mmol/L

OR

  1. Body weight less than 40Kg or greater than 100Kg

OR

  1. Unusual body habitus

What Test

  • Anti-Xa activity is the appropriate test.
  • APTT is insensitive to LMWH.
  • Blood needs to be collected 1 hour prior to and 3 hours post dose.
  • Blood should be collected by clean venipuncture and put into a coagulation tube.
  • It should be transported to the laboratory on ice.
  • 2mL in a paediatric coagulation tube is sufficient.
  • MAKE SURE THE TUBES ARE LABELLED WITH THE COLLECTION TIME.
  • This is a batched test run once a day so it is necessary to notify the lab that the test is being ordered and what time the blood is being collected.
  • Urgent tests may be discussed with the duty haematologist.

References

  1. Hirsh, J. & Levine, M. Blood 1992; 79: 1-17.
  2. Weitz, J. NEJM 1997; 337: 688-698.
  3. Bratt, G., Tornebohm, E., Lockner, D., & Bergstrom, K. Thromb. Haemostas. 1985; 54: 208-211.
  4. Duroux, P. & Beclere, A. Thromb. Haemostas. 1991; 65: 251-256.
  5. Cadroy, Y., Pourrat, J., et al Thromb. Res. 1991; 63: 385-390.
  6. Weitz, J. Personal communication 1997.

Written by:     Dr Michael Seldon, Haematology HAPS
Written:           May 1998

Last Reviewed:     10.05.2001