From the Guidelines
The next best diagnostic test for a patient with suspected Neuroleptic Malignant Syndrome (NMS) and abnormal lab results is creatine kinase (CK) measurement. This enzyme test is crucial as it typically shows marked elevation in NMS due to muscle breakdown (rhabdomyolysis), often reaching levels 3-4 times the upper limit of normal or higher 1. The patient's lab results, including a white blood cell count of 12,900, creatinine of 1.1, potassium of 5.1, bicarbonate of 19, ALT of 55, and AST of 150, suggest a possible diagnosis of NMS, and CK measurement can help confirm this diagnosis.
Additional important tests include:
- Complete blood count to assess for leukocytosis
- Comprehensive metabolic panel to assess renal function and electrolytes
- Coagulation studies
- Urinalysis to check for myoglobinuria These tests are essential because NMS is primarily a clinical diagnosis with laboratory findings supporting the clinical picture of fever, rigidity, altered mental status, and autonomic instability following exposure to dopamine-blocking agents 1.
Given that the CT of the head and chest is normal 1, the focus should be on laboratory tests that can help confirm the diagnosis of NMS. Early diagnosis through appropriate testing is critical as NMS carries significant mortality risk if not promptly identified and treated. The recent study by 1 provides a comprehensive overview of the diagnostic criteria for NMS, highlighting the importance of CK measurement in confirming the diagnosis.
From the FDA Drug Label
Clinical manifestations of NMS include hyperpyrexia, muscle rigidity, altered mental status, and autonomic instability (irregular pulse or blood pressure, tachycardia, diaphoresis, and cardiac dysrhythmia) Additional signs may include elevated creatine phosphokinase (CPK), myoglobinuria, rhabdomyolysis, and acute renal failure.
The next best diagnostic test for a patient with suspected Neuroleptic Malignant Syndrome (NMS) is to check for elevated creatine phosphokinase (CPK), as it is one of the additional signs of NMS.
- Serum creatinine should also be monitored, as acute renal failure is a possible complication of NMS.
- Given the patient's elevated potassium (5.1) and elevated creatinine (1.1), monitoring of renal function is crucial.
- However, the most directly relevant test from the options provided would be Serum creatinine, as it directly relates to the concern for acute renal failure, a complication of NMS 2.
From the Research
Diagnostic Approach for Suspected Neuroleptic Malignant Syndrome (NMS)
Given the patient's abnormal lab results, including elevated white blood cell (WBC) count, creatinine, potassium, and liver enzymes, along with a suspected diagnosis of Neuroleptic Malignant Syndrome (NMS), the next best diagnostic step is crucial for timely and appropriate management.
Consideration of Laboratory Investigations
- The patient's elevated creatinine kinase (CK) levels are not explicitly mentioned, but this is a critical marker for NMS, as highlighted in studies 3, 4, 5, 6, 7.
- Serum creatinine is already elevated at 1.1, which may indicate renal impairment, a potential complication of NMS or a side effect of certain medications.
- Electrolyte imbalance, such as hyperkalemia (potassium 5.1), and metabolic acidosis (bicarbonate 19) are also present, which can be associated with NMS or other severe conditions.
Next Diagnostic Steps
- Serum Creatine Kinase (CK): Measuring CK levels is essential, as significantly elevated CK is a common feature of NMS, although not universally present 6. This test can help in diagnosing muscle damage and is a critical component of the diagnostic criteria for NMS.
- Other Tests: While serum procalcitonin might be useful in distinguishing between infectious and non-infectious causes of fever or systemic inflammation, it is not directly relevant to the diagnosis of NMS. Electromyography and electroencephalography might be considered based on clinical presentation but are not the immediate next steps for suspected NMS.
Rationale
The primary goal is to confirm the diagnosis of NMS, given its high mortality rate if not promptly treated. The presence of altered mental status, autonomic instability, and muscle rigidity, along with laboratory evidence of muscle damage (elevated CK), are key to this diagnosis. While CK levels can be elevated due to various reasons, including physical restraint or intramuscular injections 4, in the context of suspected NMS, these levels are particularly relevant.
Given the provided options and the context of suspected NMS, ordering a Serum Creatine Kinase test would be the most appropriate next step to aid in the diagnosis, considering the critical role of CK levels in NMS, as discussed in the studies 3, 5, 7.