Urine myoglobin testing is primarily indicated for diagnosis and risk assessment of kidney injury in patients with rhabdomyolysis. However, its utility is limited by a lack of rapid and reliable results. Myoglobin reacts positively for blood by urine dipstick, which can serve as an indicator of myoglobinuria.
To evaluate the performance and value of blood and red cell measurements by urinalysis as a surrogate test for myoglobinuria in routine clinical practice.
This study is a retrospective observational study involving analysis of hemoglobin and red blood cell results by urinalysis in patients tested for urine myoglobin.
A total of 13 139 urine myoglobin results from 88 Veterans Affairs facilities during a 15-year period ending in October 2014 were evaluated. Among methods used by each laboratory, qualitative urine myoglobin tests declined from 25 of 53 (47.1%) in 2000 to 5 of 77 (6.4%) in 2013. Of 7311 tests (55.6%) performed by quantitative methods with concomitant urinalysis, 3915 (53.5%) showed negative to trace blood results, of which myoglobin was 1000 μg/L or greater in 17 (0.4%). Among 1875 (25.5%) with 3+ (large) blood results, urine myoglobin was ≥1000 μg/L in 273 of 1533 (17.8%) with hematuria (≥5 red blood cells per microliter) and 109 of 342 (31.9%) without hematuria.
Urinalysis results reliably predicted the absence of myoglobinuria and could be used to avert overtesting for urine myoglobin while also providing useful diagnostic information when urine myoglobin test results are not immediately available.
Rhabdomyolysis is a syndrome involving acute muscle injury caused by numerous conditions, such as trauma (eg, crush, burn, or electrical injuries), intense exercise, inflammatory or hereditary myopathies, snake toxins, seizures, alcohol or drug (eg, cocaine) overdose, hyperthermia, and severe hypokalemia.1,2 Muscle damage leads to increased serum levels of creatine phosphokinase and myoglobin, which serve as primary indicators for laboratory diagnosis.3,4
Serum myoglobin is normally filtered by the kidney and reabsorbed in the proximal tubules. However, with severe rhabdomyolysis, myoglobin concentrations exceed the renal clearance threshold, leading to myoglobinuria.5 Acute kidney injury is a potential complication of myoglobinuria that may cause renal tubule obstruction, vasoconstriction, and putatively, heme-induced oxidative damage. Nephrotoxic effects from myoglobin may be further potentiated by dehydration, hypotension, acidosis, or nephrotoxic drugs.6
Measurements of peak levels7 and clearance rates8,9 of serum myoglobin have been reported to have value for predicting acute renal failure in patients with rhabdomyolysis. Its detection in urine may also serve as an aid in diagnosis and assessment of severe rhabdomyolysis, and secondarily as a biomarker for risk of renal injury.10 Nevertheless, the value of testing for myoglobinuria is limited by a lack of rapid and reliable testing procedures among most clinical laboratories, analyte instability,11,12 and unclear association between urine myoglobin concentration for risk of acute renal injury.13
The peroxidase reagent used for detecting hemoglobin by dipstick testing also reacts with myoglobin, a heme-containing protein. Use of the dipstick urine blood test can provide a rapid preliminary assessment for suspected myoglobinuria but lacks analytic specificity. Furthermore, absence of red blood cells may aid in the interpretation of positive results as more likely caused by myoglobin than by hemoglobin.
Because of a lack of accurate and rapid urine myoglobin procedures, testing for blood by urinalysis is generally the only indicator of myoglobinuria available for initial management of rhabdomyolysis. However, information about the performance of this test for preliminary assessment of myoglobinuria in routine practice is limited.14 The objective of this study was to examine myoglobin testing practices and provide evidence-based support for the use of urinalysis as a surrogate test for myoglobinuria.
MATERIALS AND METHODS
A retrospective observational study was conducted involving all identifiable urine myoglobin results as well as blood and microscopic red blood cell results by urinalysis obtained from the Veterans Affairs (VA) national data warehouse. Information about urine color, casts, or other parameters was not collected. These data included the name of the VA facility, and the date results were reported. Other than general myoglobin testing method (qualitative or quantitative) no other information was available about testing site (reference laboratory or on-site), or various reagents, instruments, or analytic systems used by the 88 facilities evaluated. No patient demographics or clinical information was obtained. Urine myoglobin concentrations 1000 μg/L or greater were considered clinically significant for potential renal toxicity.15 Hematuria was defined as 5 or more red blood cells per microliter. This study was reviewed and approved by the patient privacy and research oversight officials at the authors' institution.
RESULTS
A total of 13 139 urine myoglobin results during a 15-year period (October 1999 through October 2014) were evaluated among 88 VA facilities. The median number of total myoglobin tests performed by each facility during the study period was 74, with a 10th to 90th percentile range of 14.3 to 455. A total of 5475 myoglobin tests (41.7%) were performed by qualitative methods and reported as positive or negative. The remaining 7644 myoglobin tests involved quantitative methods reported in concentration units. Qualitative myoglobin testing showed a steady decline in overall test volume and in the total number of facilities using this method relative to quantitative methods (Figure 1).
Among 7664 total quantitative urine myoglobin cases, 353 (4.5%) were excluded because of missing or incomplete urinalysis results for dipstick hemoglobin or microscopic red blood cells. Distribution of hemoglobin results by urinalysis and for urine myoglobin concentrations greater than 1000 μg/L for the remaining 7311 cases is shown in Figure 2.
Among the 3915 cases with negative or trace hemoglobin results, urine myoglobin concentration was lower than 50 μg/L in 3741 (95.6%) and 50 to 999 μg/L in 157 (4.0%). Only 17 (0.4%) were associated with urine myoglobin results 1000 μg/L or greater, whereas 4 (0.1%) were 10 000 μg/L or greater. Within the 3396 cases positive for hemoglobin by urinalysis, the proportion having myoglobinuria 1000 μg/L or greater progressively increased from 18 of 669 (2.7%) with small (1+), 83 of 852 (9.7%) with moderate (2+), and 382 of 1875 (20.4%) with large (3+) results. Furthermore, specificity for myoglobinuria at 1000 μg/L or greater was significantly associated with the absence of microscopic hematuria when dipstick blood results were in the moderate (2+), P = .002, and large (3+), P < .001, ranges but not within the small (1+) category, P = .48 (Fisher exact test; Table).
DISCUSSION
Results from this study demonstrated that urinalysis was a reliable replacement test for direct urine myoglobin measurements when hemoglobin results were negative or in trace amounts. These findings are important because of the limitations associated with the testing, clinical use, and interpretive value of urine myoglobin measurement for patients with rhabdomyolysis and risk of acute kidney injury. First, accurate urine myoglobin results are generally not available for initial evaluation because most clinical laboratories do not perform this test. For example, urine myoglobin results for qualitative and quantitative methods were reported by only 57 and 40 laboratories, respectively, in the College of American Pathologists MYG-B myoglobin proficiency survey in 2018. Furthermore, myoglobin in urine is unstable, which may affect the reliability of results from specimens that are not immediately processed or appropriately handled before testing.11,12 In contrast, urinalysis is routinely performed on demand by nearly all laboratories. Although the urinalysis hemoglobin test lacks specificity for myoglobinuria, its use as a rapid surrogate test for urine myoglobin in rhabdomyolysis is well established.4,16
There are 2 primary methods for measuring urine myoglobin. One involves qualitative methods with an ultrafiltration or precipitation step to remove hemoglobin before urine dipstick testing so that positive results for blood are presumed to be due to the presence of myoglobin. Although rapid and easy to perform, these methods have proven to be insensitive with false-negative results, as well as prone to causing false-positive results from incomplete removal of hemoglobin.17,18 This may explain the marked decline in use of these methods over time. For these reasons, qualitative urine myoglobin results were not further evaluated.
Quantitative immunoassay methods for urine myoglobin are reliable and are not affected by hematuria or hemoglobinuria. The median urine myoglobin test volume by qualitative and quantitative methods among all facilities was 74 total cases during a 15-year period, or about 5 per year. Because of low test volume, quantitative urine myoglobin testing was presumed to be performed primarily at reference laboratories. This would diminish its value because of delayed results.
Besides myoglobin load, other interacting factors, such as blood pressure and pH, affect nephrotoxicity.7 Therefore, the precise concentration of myoglobin in urine and risk of acute renal failure remain to be clarified.13 A concentration of 1000 μg/L was selected as a clinically significant cutoff value for purposes of this study.15 Although progressively higher urine concentrations are generally considered to likely cause more harm to the kidney, selection of a different level would not have affected the overall findings or conclusions. In addition, the reasons for urine myoglobin testing were unknown because clinical or patient information was not obtained. However, because urine myoglobin is primarily tested for evaluation of acute rhabdomyolysis, this was considered the most likely indication.
Cases with negative or trace amounts of blood by urinalysis but with urine myoglobin concentrations 1000 μg/L or greater were infrequent and seen in only 17 of 3917 cases (0.4%). The cause for these apparent discrepancies could not be determined, but some may have been caused by labeling, processing, analytic, or reporting errors. These results provide evidence-based support that urinalysis could be used as a prerequisite screening test to determine if a more specific test for myoglobinuria was indicated. Because about half of cases had negative to only small amounts of blood by urinalysis, a reflex testing protocol could substantially reduce overtesting and assist with management decisions by rapidly excluding the likelihood of clinically significant myoglobinuria when hemoglobin is absent or is detected in only small amounts.
Because specific information about the various urinalysis and quantitative myoglobin methods used among the 88 VA facilities evaluated was not available, variation due to bias between results is unknown and is a limitation to this type of observational study. However, these are regulated methods performed by accredited laboratories and would be expected to generally meet acceptable performance criteria. Furthermore, collective information among numerous facilities shows rare outliers and high predictive value for excluding myoglobinuria (<1000 μg/L) when urine blood is absent or in small amounts, indicating good diagnostic concordance among methods.
Results from this study also inform relative probabilities of myoglobinuria based on the amount of blood detected and presence or absence of hematuria by urinalysis. For example, lack of hematuria (<5 rbs/μL) and large (3+) urine blood by urinalysis dipstick testing was associated with a 31% likelihood of high (≥1000 μg/L) urine myoglobin concentrations. This predictive information may be useful for the initial assessment of patients with rhabdomyolysis and perhaps for monitoring urine myoglobin clearance.
In summary, this large observational study provides evidence-based support for the role of urinalysis in testing for myoglobinuria. Measurement of urine myoglobin is unnecessary in cases with absent or only small amounts of blood by urinalysis which reliably excludes the presence of clinically significant myoglobinuria. Conversely, the presence of increasing amounts of blood by urinalysis progressively raises the probability of myoglobinuria, especially in the absence of hematuria. When urine myoglobin cannot be promptly and accurately measured, its value is limited to that of a confirmatory test, with marginal clinical impact when used for diagnosis and management of rhabdomyolysis compared to that provided by urinalysis results.