Weight assessment in cardiac patients: implications for prescription of low molecular weight heparin

Many drugs such as low molecular weight heparin (LMWH) are administered at "patient weight adjusted" doses. Obtaining an accurate measurement of a patient's weight may not always be possible. The aim of this study was to assess patterns and accuracy of weight estimation and implications for drug dosing.

The study comprised three parts: (1) inpatient weight documentation was reviewed over a 4 week period (January 2008); (2) a questionnaire was distributed to healthcare staff; (3) healthcare staff were asked to estimate the weight of patients. These estimates took place in three locations: the coronary care unit, cardiac catheterisation laboratory, and the cardiac outpatient department.

(1) In 385 patient notes, only 192 (49.9%) had a record of the patient's weight. The dose of LMWH was correct only 51% of the time. (2) Doctors were more likely to estimate a patient's weight than nurses (85 vs 51%, p = 0.003). (3) 50 healthcare staff made 533 weight estimations on 182 patients. There was a tendency to overestimate the weight of lighter patients and underestimate the weight of heavier patients (p<0.001). Patients were more accurate than healthcare staff at estimating their weight (80% vs 39%, p<0.001) and female patients were more likely to be accurate than men (62% vs 44%, p = 0.035).

In our institution weight estimation occurs and may result in inaccurate prescription of LMWH. Estimating a patient's weight should be discouraged but if necessary the patient reported weight is likely to be most accurate. Unless there is significant investment in improved technology to allow obese or acutely unwell patients to be weighed, the dangerous practice of weight estimation is likely to continue.