What lab tests should I order to evaluate creatinine, GFR (Glomerular Filtration Rate), potassium, platelets, AST (Aspartate Aminotransferase), and ALT (Alanine Aminotransferase) levels?

Laboratory Tests for Evaluating Creatinine, GFR, Potassium, Platelets, AST, and ALT

To properly evaluate creatinine, GFR, potassium, platelets, AST, and ALT levels, you should order a comprehensive metabolic panel (CMP) plus a complete blood count (CBC).

Recommended Laboratory Tests

Basic Tests

• Complete Blood Count (CBC) - Includes platelet count assessment 1
• Comprehensive Metabolic Panel (CMP) - Includes:
  • Creatinine 1
  • Electrolytes (including potassium) 1
  • Liver enzymes (AST and ALT) 1

GFR Assessment

• eGFR calculation - Should be automatically calculated by the laboratory based on serum creatinine, age, gender, and race 2
  • Most laboratories use the MDRD or CKD-EPI equation for this calculation 2
  • No need to order a separate test for GFR as it is derived from creatinine 2

Clinical Importance of These Tests

Liver Function Assessment

• AST and ALT are essential markers for evaluating hepatocellular injury 1
• These enzymes should be routinely measured as part of liver blood test panels 1
• ALT is more specific for liver injury than AST, which can also be elevated in cardiac, skeletal muscle, and kidney disorders 1
• The AST/ALT ratio can be useful in evaluating certain liver conditions (ratio >2 suggests alcoholic hepatitis) 1

Kidney Function Assessment

• Serum creatinine is the primary marker for estimating kidney function 1, 2
• eGFR provides a more accurate assessment of kidney function than creatinine alone 2
• Serum creatinine should not be used in isolation to assess kidney function 2
• Potassium levels are important to monitor in patients with kidney disease 1

Platelet Assessment

• Platelet count is crucial for evaluating liver fibrosis and portal hypertension 1
• Low platelet count may indicate advanced liver disease or other hematologic disorders 1

Important Considerations

For Liver Assessment

• When evaluating for liver fibrosis, consider that AST, ALT, and platelet count are components of non-invasive fibrosis scores such as FIB-4 1
• FIB-4 is calculated using: age (yr) × AST (IU/L)/platelet count (10^9/L × [ALT (IU/L)]^1/2) 1
• For comprehensive liver evaluation, consider adding:
  • Albumin 1
  • Bilirubin (total and direct) 1
  • Prothrombin time (PT) 1

For Kidney Assessment

• 24-hour urine collections for creatinine clearance are generally not necessary unless the patient has exceptional dietary intake or muscle mass 2
• Consider adding BUN (blood urea nitrogen) to the panel for a more complete assessment of kidney function 1, 3

Common Pitfalls to Avoid

• Do not rely solely on serum creatinine to assess kidney function; eGFR provides a more accurate estimate 2
• Be aware that AST can be elevated due to non-hepatic causes (muscle injury, heart disease) 1, 4
• Recognize that platelet count can be affected by many conditions unrelated to liver disease 1
• Remember that GFR estimates may be less accurate in certain populations, including those with exceptional muscle mass, malnutrition, or vegetarian diets 2

By ordering a CBC and CMP, you will obtain all the necessary laboratory values to assess creatinine, GFR, potassium, platelets, AST, and ALT levels in a single comprehensive panel 1.