Should I order additional labs to rule out underlying conditions?

Ordering Additional Laboratory Tests: A Targeted Approach

Additional laboratory testing should only be performed when there is a specific clinical indication based on history, physical examination findings, or initial test results that suggest an underlying condition requiring further investigation. 1

When to Order Additional Tests

Laboratory testing should follow a strategic approach based on clinical suspicion rather than routine panels:

1. Iron Deficiency Workup:

   • If anemia is present, diagnose iron deficiency when ferritin < 45 ng/mL
   • Additional tests needed only if underlying inflammation or chronic kidney disease is present
   • For suspected celiac disease, order serologic testing first, followed by small bowel biopsy only if positive 1

2. Asymptomatic Microhematuria Evaluation:

   • If microhematuria is confirmed (≥3 RBCs per HPF on properly collected specimens), assess renal function with:
     • Estimated GFR
     • Creatinine
     • BUN 1

3. Liver Function Abnormalities:

   • Do not simply repeat abnormal liver tests without a diagnostic plan
   • 84% of abnormal liver tests remain abnormal when retested after 1 month
   • Determine the cause rather than monitoring without investigation 1

Tests to Avoid Without Specific Indications

• Routine laboratory panels for patients without specific symptoms or risk factors 2
• Repeated liver tests without a plan to determine etiology 1
• Routine diagnostic imaging for common conditions without red flags 1, 3

Clinical Decision Algorithm

1. Identify the specific clinical question:

   • What diagnosis am I trying to confirm or exclude?
   • Will the test result change management?
   • Is there a pre-test probability that justifies testing? 4

2. Consider test characteristics:

   • Sensitivity and specificity for the condition of concern
   • Positive and negative predictive value in your specific patient population
   • Cost and potential harms of testing 3

3. Order tests only when:

   • They have reasonable diagnostic yield
   • Are low risk
   • Are reasonable in cost
   • Will improve patient management 1

Common Pitfalls to Avoid

• Defensive testing: Ordering tests "just to be sure" without clear indications increases costs and can lead to false positives 2
• Routine panels: Up to 60-70% of laboratory tests ordered in high-throughput settings may be clinically unnecessary 2
• Failure to consider pre-test probability: Tests have the most value when there is genuine diagnostic uncertainty 4, 5
• Ignoring context: The extent of laboratory abnormality is not necessarily a guide to clinical significance; context matters more than the degree of abnormality 1

By following a targeted approach to additional laboratory testing based on specific clinical indications, you can provide high-quality care while avoiding unnecessary testing, reducing costs, and preventing potential harms from false positives or incidental findings.