Serum myoglobin immunoassays: obsolete or still clinically useful?

Rhabdomyolysis is conventionally defined as a clinical syndrome attributable to massive skeletal muscle injury, with consequent leakage of many intracellular and often toxic components (e.g. potassium and myoglobin among others) into the blood. The epidemiologic data attests that rhabdomyolysis is a relatively frequent condition in the adult populations, with as many as 26,000 cases annually reported in the US [1]. The most frequent causes of rhabdomyolysis include traumatic muscle injury, hereditary enzyme disorders, drugs, toxins, endocrinopathies, malignant hyperthermia, neuroleptic malignant syndrome, heatstroke, hypothermia, severe electrolyte disorders, diabetic ketoacidosis, severe hyperthyroidism, as well as bacterial or viral infections [2]. Although the overall mortality for rhabdomyolysis depends on many factors such as the etiology, comorbid conditions and complications, the death rate remains high, comprised between 5% and 10%. Notably, the most frequent and severe complication of rhabdomyolysis is acute kidney injury (AKI), the presence of which contributes to enhancing the overall mortality up to 30%, approximating 60% in intensive care unit (ICU) patients [3].

The diagnosis of rhabdomyolysis is prevalently based on clinical and laboratory data. For decades the laboratory diagnostics of rhabdomyolysis has been essentially based on the assessment of the enzymatic activity of creatine kinase (CK; EC 2.7.3.2), which has since been regarded as the most sensitive and predictive biomarker in patients with this condition. The measurement of urinary myoglobin, an additional biomarker of muscle damage, has been gradually abandoned due to the many clinical and technical issues related to its analytical assessment [4, 5]. After a decade of magnificence, the measurement of serum myoglobin has also been progressively forsaken in clinical laboratories, due to the introduction of more sensitive cardiospecific troponin immunoassays, which have made the assessment of myoglobin virtually useless for diagnosing acute coronary syndrome [6]. Nevertheless, evidence which has merged over the past 5 years attests that the measurement of myoglobin in serum (or plasma) may still have an important role in patients with rhabdomyolysis, especially for predicting the development of AKI.

Nearly 5 years ago, Kasaoka et al. studied 30 patients with rhabdomyolysis (the leading causes were trauma, burns and ischemia), 12 of whom developed AKI [7]. In multivariate logistic regression analysis, the peak value of serum myoglobin was found to be a much better predictor of AKI [odds ratio (OR), 1.19; 95% confidence interval (CI), 0.99–1.43] than the peak value of CK (OR, 0.99; 95% CI, 0.92–1.06). The overall diagnostic performance of serum myoglobin was noteworthy [i.e. exhibiting an area under the curve (AUC) of ~0.90].

El-Abdellati et al. retrospectively studied 1769 adult ICU patients, using hemodialysis and mortality as outcome variables [8]. The risk for the onset of AKI was found to be the highest for serum myoglobin (OR, 7.87; 95% CI, 4.60–13.85; p<0.001), followed by CK (OR, 2.21; 95% CI, 1.45–3.38; p<0.001) and urinary myoglobin (OR, 1.61; 95% CI, 1.21–2.13; p=0.001). Serum myoglobin was found to be the best predictor of AKI development (AUC, 0.790), whereas the efficiency of CK (AUC, 0.759) was also preceded by that of urinary myoglobin (AUC, 0.766). Two additional and important aspects were highlighted in this study. First, elevated CK values with concomitant normal serum myoglobin concentration were not predictive of AKI (p=0.882). Then, the peak of CK values (72±145 h) was reached significantly later than that of both serum (30±87 h) and urinary (33±77 h) myoglobin.

Premru et al. studied the incidence of myoglobinuric AKI and the need for hemodialysis in 484 patients with suspected rhabdomyolysis [9]. The value of CK significantly correlated with the severity of rhabdomyolysis, but such a correlation was absent in patients with myoglobinuric AKI. More specifically, a better association was found between serum creatinine and serum myoglobin (r=0.33; p<0.001) than between serum creatinine and CK (r=0.15; p=0.01). Interestingly, when patients were grouped by CK values, the serum creatinine values did not differ among groups either clustered according to the CK cut-off or CK quartiles, whereas serum creatine values were significantly different among groups clustered according to peak serum myoglobin values.

Additional support to the use of serum myoglobin assessment for predicting AKI emerged from the study of Chen et al. [10], who studied 202 patients admitted to the emergency department with a definite diagnosis of rhabdomyolysis. The leading causes of rhabdomyolysis were trauma and infections, both conditions accounting for approximately half of all patients. Interestingly, CK values measured at different time points (i.e. from admission, up to the fifth day afterward) did not efficiently discriminate patients with AKI from those without, whereas the diagnostic performance of the initial serum myoglobin measurement was excellent (i.e. AUC, 0.72).

These findings were confirmed by a recent meta-analysis of 18 published studies [11], which concluded that AKI occurrence was significantly predicted by CK value in patients with crush-induced rhabdomyolysis (OR, 14.7; 95% CI, 7.63–28.52; p=0.001) but not in those with other causes of rhabdomyolysis (OR, 0.99; 95% CI, 0.92–1.06; p=0.08).

Notably, some case reports have also been published, highlighting the major importance of measuring serum myoglobin other than CK for predicting the development of AKI. Yong et al. published the paradigmatic case of a 49-year-old man who was admitted to the emergency department with fever, chills, rigors and generalized myalgia, and who was finally diagnosed with malaria complicated by rhabdomyolysis [12]. Surprisingly, the values of serum and urinary myoglobin were found to be substantially increased (138 μg/L and 194 μg/L, respectively), whereas the concentration of CK remained always within the normal range (i.e. 99 U/L).

Beside the existence of reliable clinical evidence, there are additional reasons that would favor the assessment of serum myoglobin in patients with rhabdomyolysis. First, it is now clearly established that myoglobin-induced renal toxicity plays a crucial role in rhabdomyolysis-associated kidney damage, mainly by enhancing oxidative stress, triggering a sustained inflammatory response and endothelial dysfunction, but also mediating vasoconstriction and apoptosis (Figure 1) [13]. The involvement of myoglobin in renal injury has also been confirmed by a recent immunohistochemistry study, revealing the presence of myoglobin casts in approximately 50% of renal biopsies performed in patients with morphologically suspicious or atypical casts [14]. Interestingly, positive staining for myoglobin was identified in all casts, but was also detected in proximal tubular epithelial cells without casts and in dehisced epithelial cells, thus confirming the active role of myoglobin in triggering renal injury and dysfunction.

Figure 1:

Pathogenesis of myoglobinuric acute kidney injury after rhabdomyolysis.

There is now undeniable evidence that myoglobinuric AKI is the most severe complication in patients with rhabdomyolysis, wherein the development of kidney damage is the leading cause of mortality and long-term morbidity (i.e. hemodialysis dependence). Thus, the availability of reliable predictive biomarkers of AKI is crucial for early and effective patient management. Despite the measurement of CK remains the gold standard for identifying and monitoring muscle injury, especially in the sport setting [15], mounting evidence suggests that the role of serum (or plasma) myoglobin should be reassessed for predicting AKI in patients with rhabdomyolysis. There are at least three essential reasons for supporting this theory. First, the pathophysiologic role of myoglobin is now undeniable in the development of myoglobinuric AKI (Figure 1), so that its measurement would provide a reasonable clue to mirror (direct or indirect) damage to the kidneys. The rapid kinetics of serum myoglobin would also enable a much faster prediction of AKI than the assessment of CK, thus enabling a more timely and efficient patient management. Last but not least, longitudinal monitoring of serum myoglobin values may permit to more closely mirror disease activity and therapeutic efficiency.

Although the assessment of CK may remain the biochemical gold standard for identifying and predicting the severity of muscle injury for a long time, the role of serum myoglobin immunoassays should be reassessed in modern clinical laboratories, at least for predicting the risk of developing AKI in patients with rhabdomyolysis.