Blood Tests for Detecting a Kidney Infection
The most important blood tests for detecting a kidney infection are a complete blood count (CBC) with differential and measurements of blood urea nitrogen (BUN) and serum creatinine. 1, 2
Primary Blood Tests
Complete Blood Count (CBC): Essential for evaluating suspected kidney infections, as it can reveal an elevated white blood cell count and neutrophil count, which are indicators of bacterial infection 1
Blood Urea Nitrogen (BUN): Measures the amount of nitrogen in the blood that comes from the waste product urea; elevated levels may indicate decreased kidney function due to infection 2
Serum Creatinine: More specific marker of kidney function than BUN; helps assess the extent of kidney impairment caused by infection 2, 3
Electrolytes: Measurement of sodium (Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), and chloride (Cl-) levels can help evaluate kidney function and acid-base balance 1
Additional Blood Tests
C-Reactive Protein (CRP): An inflammatory marker that may be significantly elevated (≥50 mg/L) in kidney infections, particularly in cyst infections 1
Blood Cultures: Should be obtained when a kidney infection is suspected to identify the causative organism and guide antibiotic therapy 1
Bicarbonate Levels: Help assess acid-base balance, which may be disrupted in kidney infections 1
Interpretation and Clinical Context
A white blood cell count >11 × 10^9/L combined with elevated CRP is highly suggestive of kidney infection when clinical symptoms are present 1
The BUN-to-creatinine ratio can provide additional diagnostic information - an elevated ratio may suggest pre-renal causes, while a normal ratio with elevated absolute values suggests intrinsic renal disease 2
Blood test results should always be interpreted in conjunction with urinalysis findings, particularly the presence of white blood cells, red blood cells, and bacteria in urine 1
Important Considerations
Blood tests alone cannot definitively diagnose a kidney infection but must be combined with clinical symptoms (fever, flank pain) and urinalysis 1
In patients with suspected kidney infection, blood tests should be performed before starting antibiotic therapy to establish baseline values 1
Frequent monitoring of blood counts, electrolytes, BUN, and creatinine is recommended in patients with diagnosed kidney infections to detect early manifestations of complications 1
For patients with chronic kidney disease who develop acute kidney infections, interpreting blood test results may be more challenging and requires comparison to baseline values 1
Newer Biomarkers
Newer blood biomarkers such as serum cystatin C may provide more accurate assessment of glomerular filtration rate than traditional markers like creatinine and BUN in the setting of acute kidney injury from infection 4
Research is ongoing for more sensitive and specific biomarkers that could detect kidney infection and injury earlier than conventional tests 4, 5