Weighing in on Weight Estimation for rtPA Dosing

When a stroke patient rolls into the emergency department, there is limited time window to administer recombinant tissue plasminogen activator (rtPA).

When a stroke patient rolls into the emergency department, there is limited time window to administer recombinant tissue plasminogen activator (rtPA).

This narrow therapeutic window often pressures hospital staff to administer the potentially brain-saving medication as early as possible. Often, administration is hastened by estimating patient weight as a basis for calculating the critical 0.9 mg/kg weight-based dose.

However, one question remains: if the patient’s weight is not appropriately estimated, what consequences can this have on the patient?

A recently published retrospective analysis is the largest study to date on this topic.1 The study analyzed 242 patients with stroke who received an rtPA dose based on an estimated bodyweight.

After determining actual patient weight, investigators calculated the dosing error. Primary outcome measures included improvement in National Institute of Health Stroke Scale (NIHSS) and modified Rankin Scale (mRS) scores.

Alarmingly, the investigators found that 19.7% of patients received an inappropriate dose of rtPA, defined as a deviation of at least 10% from the appropriate calculated dose. Clinicians in the study significantly overdosed the lightest patients and underdosed the heaviest ones.

Dosing errors did not significantly affect mRS scores or rate of intracranial hemorrhage; however, NIHSS score improvement was greater in the subset of patients with the lightest weight compared with the subset of patients with the heaviest weight (P=0.048). These results conflict with earlier findings that cited an increased trend of intracranial hemorrhage in patients who received higher doses due to overestimation of weight, with no findings of decreased efficacy in patients who received lower doses due to underestimation of weight.2

Of note, heavier patients in the study had higher baseline NIHSS scores, implying that they had more severe strokes (P<0.001).1 Therefore, it remains unclear whether these patients had poorer outcomes due to underdosing, higher stroke severity, or a combination of the 2.

What is clear is that estimating patient weight for rtPA dosing is unreliable. An error rate of nearly 20% is alarming and consistent with prior findings.

One study found that an emergency medicine cohort of physicians, paramedics, nurses, and medical students appropriately estimated weight within 5 kg in only 28.1% of patients.3 Additional research from the WAIST-Study revealed an error rate of 33% and a trend of worse outcomes in underdosed stroke patients.4

Based on the consistent erroneous findings for weight estimation in stroke patients, beds with the capability to measure weight should be used in the emergency department whenever possible in order to minimize weight-based dosing errors and their potentially negative impact on patient outcomes.

References

1. Barrow T, Khan MS, Halse O, et al. Estimating weight of patients with acute stroke when dosing for thrombolysis. Stroke. 2016;47(1):228-231.

2. Messé SR, Tanne D, Demchuk AM, et al. Dosing errors may impact the risk of rt-PA for stroke: the Multicenter rt-PA Acute Stroke Survey. J Stroke Cerebrovasc Dis. 2004;13(1):35-40.

3. Hall WL, Larkin GL, Trujillo MJ, et al. Errors in weight estimation in the emergency department: comparing performance by providers and patients. J Emerg Med. 2004;27(3):219-224.

4. Breuer L, Nowe T, Huttner HB, et al. Weight approximation in stroke before thrombolysis: the WAIST-Study: a prospective observational "dose-finding" study. Stroke. 2010;41(12):2867-2871.