Article

Urine dipstick testing to rule out rhabdomyolysis in patients with suspected heat injury

Brief Report

Urine dipstick testing to rule out rhabdomyolysis in patients with suspected heat injury?

Scott E. Young DOa, Michael A. Miller MDa,b,c,?, Martin Docherty MDa

aDepartment of Emergency Medicine, Darnall Army Medical Center, Ft. Hood, TX 76544, USA

bCentral Texas Poison Control Center, Temple, TX 76508, USA

cDepartment of Emergency Medicine, Tripler Army Medical Center, HI 96859, USA

Received 21 June 2008; revised 23 June 2008; accepted 24 June 2008

Abstract

Background: Heat injury is a common, potentially life-threatening medical condition. In austere or mass-casualty conditions an easy to use, sensitive screening test could be a valuable tool to care providers and evacuation planners.

Objective: The objective of the study was to determine if a simple urine dipstick test for blood is sensitive for detection of rhabdomyolysis in the suspected heat injury patient.

Material and Methods: A convenience sample of patients presenting to a military community hospital Emergency Department during summer months with a presenting complaint consistent with suspected heat injury had urine dipstick testing performed for blood and compared with the results of formal urinalysis and serum creatine kinase.

Results: 60 patients were enrolled in the study, seven had creatine kinase levels greater than 1000U/L, 14 had levels greater than 500U/L, and 26 had levels greater than 250 U/L. Using 1000U/L, urine dipstick testing had a sensitivity of 14% and a specificity of 85%.

Conclusions: Urine dipstick testing for blood is not a useful screening test for rhabdomyolysis in patients suspected to have significant heat injury.

(C) 2009

Introduction

Background

Heat injury is a potential threat to participants involved in sporting events, outdoor recreational activities, Mass gatherings, as well as operations involving law enforce-

? This work has not been published elsewhere. This study is the opinion of the authors and does not necessarily represent the opinions of the US Army or the Department of Defense.

* Corresponding author. Department of Emergency Medicine, Tripler

Army Medical Center, Hawaii.

E-mail address: [email protected] (M.A. Miller).

ment, fire service, and military personnel. Heat injury can be life threatening, illustrated by the 8015 deaths attributed to heat exposure in the United States from 1979 to 1999 [1]. The US Army reported almost 1000 cases of acute heat injury in 2000 [2]. In addition to individuals with heat injury, large numbers of patients can present during a short period during mass gatherings, such as during the Hajj, where one study reported more than 1700 deaths during a single season [3]. The evaluation for heat injury of a large number of patients in a short period as well as individual patients in a busy emergency department (ED) can be time consuming and use a number of clinical resources.

Rhabdomyolysis is one of the more concerning complica- tions of heat injury and occurs when muscle fibers are damaged,

0735-6757/$ – see front matter (C) 2009 doi:10.1016/j.ajem.2008.06.020

876 S.E. Young et al.

leading to a release of the enzyme creatine kinase (CK) [4]. Myoglobin is also released during Muscle breakdown. It rises in the serum about 2 hours after the initiating event and reaches an early renal threshold at which time it can be detected in the urine. However, even in the presence of clinically significant rhabdomyolysis, myoglobin may be undetectable in urine until after 6 hours [4]. The hallmark of myoglobinuria is tea- colored urine. However, smaller levels of myoglobinuria may not be visually detectable. A well-known test for myoglobinuria involves the urine dipstick (UD) test. Orthotoluidine on the UD test strip reacts to the heme found in myoglobin as well as hemoglobin [4]. Classic clinical teaching states that rhabdo- myolysis can be manifested by a UD that is positive for blood and a Urine sample negative for red blood cells.

Importance

The use of the UD as a screen for rhabdomyolysis is a potentially valuable tool for use in the ED as well as in the prehospital setting. Appropriate application of the UD could potentially save time and resources in both settings. In the prehospital setting, a rapid screen could help determine whether or not a patient should be allowed to continue with activity involving exertion and heat exposure. In the ED, the UD could help facilitate rapid disposition of patients with suspected heat injury.

Goals

The goal of this study is to determine if a simple UD is sensitive for rhabdomyolysis in the patient with suspected heat injury.

Methods

Setting

This prospective study was performed on a convenience sample of patients that presented to a military community hospital ED during the summer months of 2003 and 2004. The protocol was approved by the institutional review board for the study hospital’s parent institution.

Method of selecting participants

Patients were included if they were 18 years or older and the treating physician chose to evaluate them for suspected heat injury based on their chief complaint and presenting symptoms. If a patient was included in the study, the treating physician ordered a serum CK, UD, and urinalysis along with any other testing they felt was appropriate. No change in the prehospital or ED-based treatment plan was made for patients included in the study.

Methods of measurement

Urinalysis and CK were processed in the usual fashion. Urine dipstick was interpreted electronically by a Chemstrip Criterion II Urine Analyzer (Roche Diagnostics, Indianapo- lis, Ind). The results of these studies were then entered into the hospitals’ computerized database system as is the standard operating procedure of the hospital laboratory.

Data collection and processing

The treating physician recorded patient identification information if they met the criteria for inclusion in the study, which the investigators then used to review the computer- based laboratory results at a later time. These data were entered into Microsoft Excel 2003 (Microsoft Corporation, Redmond, Wash) spreadsheet for analysis.

Sensitivity, specificity, and likelihood ratio calculations were performed using Web-based statistics software.

Outcome measures

Primary outcome measures were serum CK and the presence or absence of blood as determined by UD.

Results

Sixty patients evaluated for heat injury in the ED were enrolled during the summer months of the 2-year study period. Seven patients had a CK greater than 1000 U/L, 14 had a CK greater than 500 U/L, and 26 patients had a CK greater than 250 U/L. Two patients had very significant CK elevations of 52 677 and 20 908 U/L. Of the 60 patients, 9 had a UD positive for blood. Of those, 1 had a CK greater than 1000 U/L (52 677 U/L), and 2 patients had a CK greater than 500 U/L. The patient with a CK of 20 908 U/L had a negative UD.

With 1000 U/L as the definition of rhabdomyolysis, UD had a sensitivity of 14.3% (95% confidence interval [CI], 0.8-57.9) and a specificity of 84.9% (95% CI, 71.9-92.8), a positive likelihood ratio of 1.0 (95% CI, 0.1-6.5), and a negative likelihood ratio of 1.0 (95% CI, 0.7-1.4). If the CK used to define rhabdomyolysis was lowered to 500 U/L, UD had a sensitivity of 14.3% (95% CI, 2.5-43.8), a specificity

Table 1 Performance of UD in the evaluation of patients with suspected heat injury

Serum CK (U/L)

<=500

N500

<=1000

N1000

UD

Positive

7

2

8

1

Negative

39

12

45

6

Urine dipstick testing to rule out rhabdomyolysis 877

of 84.8% (95% CI, 70.5-93.1), a positive likelihood ratio of

0.9 (95% CI, 0.2-4.0), and a negative likelihood ratio of 1 (95% CI, 0.8-1.3). If this threshold was lowered further to 250 U/L, UD had a sensitivity of 19.2% (95% CI, 7.3-40.0),

a specificity of 88.2% (95% CI, 71.6-96.2), a positive likelihood ratio of 1.6 (95% CI, 0.5-5.5), and a negative likelihood ratio of 0.9 (95% CI, 0.8-1.1) (see Table 1).

Limitations

The patients enrolled in this study were mostly young, active duty military personnel. They likely had few comorbidities and performed moderate exercise on a routine basis. No patients had significant renal failure, and only 2 had Severe rhabdomyolysis. The limited number of patients enrolled in the study clearly affects the wide CIs noted with the sensitivities.

The investigators did not control for treatment received before presentation to the ED. Some of the included patients received intravenous fluids from Prehospital personnel before ED arrival. They may have also received intravenous or oral fluids in the ED before giving a urine sample. The goal of this study was to evaluate the use of UD in an uncontrolled population, similar to the population that would be faced in day-to-day practice. It would be difficult to get a urine sample from all patients before beginning fluid therapy for heat injury, and the impact of hydration on the urine results is unclear.

Although the textbook definition of rhabdomyolysis is a CK greater than 5 to 10 times normal, this does not clearly define when rhabdomyolysis becomes clinically significant [5]. The purpose of this study was not to detect clinically significant rhabdomyolysis with a UD, it was only to determine whether or not UD has the sensitivity to screen for an elevated CK.

Discussion

Rhabdomyolysis is a potentially significant complication of heat induced injury and may lead to acute renal failure,

arrhythmia, and death. The results of this limited study do not support the classic assumption that a positive UD in the absence of red blood cells on urinalysis represents rhabdomyolysis. The exact value at which rhabdomyolysis becomes a clinically significant concern is unclear, but using this somewhat limited patient population, UD does not appear to be sensitive for CKs of 1000 or lower.

Further research is clearly indicated to identify in which patients rhabdomyolysis is clinically significant, and whether or not UD would be useful as a screening test in this population. Future investigations should also look to expand the population evaluated to include the elderly, patients with significant comorbidities, and those with renal insufficiency.

Conclusion

Urine dipstick is not a sufficient screening test for rhabdomyolysis in patients being evaluated for suspected heat injury. A patient that presents in the prehospital or ED setting with signs and symptoms consistent with significant heat injury should undergo a thorough clinical evaluation, with history and physical and laboratory examination to include a serum CK.

References

  1. Centers for Disease Control and Prevention. Heat related deaths-four states, July-August 2001, United States, 1979-1999. MMWR 2002;51

(26):567-70.

  1. Medical Surveillance Monthly Report. Heat injuries-U.S. Army, 1998- 2000. Vol 7 (3) 2001:2-7. No. 3, March 2001.
  2. Al Ghamdi SM, Akbar HO, Qari YA, et al. Patterns of admission to hospitals during Muslim pilgrimage (Hajj). Saudi Med J 2003;24: 1073-6.
  3. Penn AS. Myoglobinuria. In: Engel AG, Banker BQ, editors. Myology. New York: Mcgraw-Hill; 1986. p. 1785-805.
  4. Tintenalli JE, Kelen GD, Stapczynski JS. Emergency medicine: a comprehensive study guide. 6th ed. New York (NY): McGraw-Hill; 2004. p. 1749.

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