From the Guidelines
To differentiate between Acute Kidney Injury (AKI) and Chronic Kidney Disease (CKD), order serum creatinine, blood urea nitrogen (BUN), electrolytes (sodium, potassium, chloride, bicarbonate), complete blood count, and urinalysis with microscopy as baseline tests. For AKI evaluation, add urine sodium, fractional excretion of sodium (FENa), urine osmolality, and kidney ultrasound to assess for obstruction 1. Serial creatinine measurements over hours to days help track AKI progression. For CKD assessment, include estimated glomerular filtration rate (eGFR), urine albumin-to-creatinine ratio, calcium, phosphorus, parathyroid hormone, and vitamin D levels. Consider renal ultrasound to evaluate kidney size and structure, as small kidneys suggest chronicity. Previous laboratory values are crucial - stable elevated creatinine over months suggests CKD, while a rapid rise indicates AKI 1. AKI typically develops over hours to days with potentially reversible kidney damage, while CKD represents progressive loss of kidney function over months to years. Some patients may have AKI superimposed on CKD, requiring comprehensive evaluation of both acute and chronic parameters. The timing of laboratory abnormalities helps distinguish between these conditions and guides appropriate management strategies.
Key laboratory tests for AKI and CKD include:
- Serum creatinine
- Blood urea nitrogen (BUN)
- Electrolytes (sodium, potassium, chloride, bicarbonate)
- Complete blood count
- Urinalysis with microscopy
- Urine sodium
- Fractional excretion of sodium (FENa)
- Urine osmolality
- Kidney ultrasound
- Estimated glomerular filtration rate (eGFR)
- Urine albumin-to-creatinine ratio
- Calcium
- Phosphorus
- Parathyroid hormone
- Vitamin D levels
It is essential to consider the clinical context and patient history when interpreting laboratory results, as some patients may have underlying CKD that predisposes them to AKI 1. The KDIGO consensus conference recommends a classification system that differentiates AKD without AKI and AKD with AKI, and acknowledges the importance of cause-specific therapy 1. By ordering the appropriate laboratory tests and considering the clinical context, healthcare providers can accurately diagnose and manage AKI and CKD, improving patient outcomes and reducing morbidity and mortality.
From the Research
Laboratory Tests for AKI and CKD
To differentiate between Acute Kidney Injury (AKI) and Chronic Kidney Disease (CKD), several laboratory tests can be ordered, including:
- Serum creatinine level: This is a crucial test for diagnosing AKI, as defined by an increase in serum creatinine of 0.3 mg/dL or more within 48 hours, or an increase of 1.5 times or more of the baseline level within 7 days 2
- Urinalysis with microscopy: This test can help narrow down the differential diagnosis of AKI and identify underlying causes such as nephrotoxic drugs or systemic illnesses 2, 3
- Complete blood count (CBC): This test can help identify underlying conditions that may be contributing to AKI, such as anemia or infection 3
- Fractional excretion of sodium: This test can help determine the cause of AKI, such as prerenal, intrinsic renal, or postrenal 3
- Ultrasonography of the kidneys: This test can help rule out obstruction and identify other underlying causes of AKI, particularly in older men 3
- Electrolyte panel: This test can help identify electrolyte imbalances that may be contributing to AKI, such as hyperkalemia or hypokalemia 3
Additional Considerations
When ordering laboratory tests for AKI and CKD, it is essential to consider the patient's medical history, physical examination, and risk factors for kidney disease. The choice of reference serum creatinine is also crucial in defining AKI, and pre-hospital creatinine values can be useful in determining reference creatinine 4. Additionally, the use of novel biomarkers, such as those that detect cellular stress or damage, may help improve the diagnosis and management of AKI 5.