What are the common blood tests in primary care?

Common Blood Tests in Primary Care

The most common blood tests in primary care include complete blood count (CBC), comprehensive metabolic panel, lipid panel, thyroid function tests, hemoglobin A1c, and disease-specific tests based on risk factors or symptoms. These tests provide essential information for diagnosis, screening, and monitoring of various conditions.

Complete Blood Count (CBC)

• CBC is one of the most commonly performed blood tests in primary care, providing information about red blood cells, white blood cells, and platelets 1
• Components include hemoglobin, hematocrit, red blood cell count, white blood cell count with differential, platelet count, and red cell indices (MCV, MCH, MCHC) 1, 2
• Essential for diagnosing anemia, hematological cancers, infections, acute hemorrhagic states, allergies, and immunodeficiencies 1
• Also used for monitoring side effects of certain medications 1
• Point-of-care testing for CBC parameters is becoming available in primary care settings, allowing for rapid results 3

HIV-Related Testing

• For patients with HIV, routine testing includes CD4 cell count, HIV viral load, complete blood count, and comprehensive metabolic panel 4
• HIV-specific tests include HIV serology (if diagnosis not previously confirmed), CD4 cell count and percentage, plasma HIV RNA (viral load), and HIV resistance testing 4
• Other laboratory tests for HIV patients include:
  • Complete blood count with differential
  • Liver function tests (ALT, AST, total bilirubin, alkaline phosphatase)
  • Total protein/albumin
  • Electrolytes, BUN/creatinine
  • Fasting lipid profile and blood glucose
  • Urinalysis 4

Liver Function and Fibrosis Assessment

• ALT, AST, and platelet count should be part of routine investigations in primary care for patients with suspected liver disease 4
• Simple non-invasive fibrosis tests like FIB-4 can be calculated using these parameters to assess liver fibrosis risk 4
• Automatic calculation and systematic reporting of simple non-invasive fibrosis tests is recommended in populations at risk of liver fibrosis (individuals with metabolic risk factors and/or harmful alcohol use) 4
• Non-invasive fibrosis tests should be used for ruling out rather than diagnosing advanced fibrosis in low-prevalence populations 4

Gastrointestinal Testing

• Fecal calprotectin is a useful screening tool for patients aged 16-40 presenting with chronic diarrhea and symptoms that may be consistent with either IBD or IBS 4
• For patients with suspected IBD, initial blood tests should include full blood count, urea & electrolytes, CRP, and coeliac screen 4

Cardiovascular and Metabolic Disease Assessment

• CBC components can help predict risk of cardiovascular disease, atherosclerosis, type 2 diabetes, and metabolic syndrome 5
• Useful markers include white blood cell count, neutrophil-to-lymphocyte ratio, hemoglobin, mean corpuscular volume, red cell distribution width, platelet count, mean platelet volume, and platelet-to-lymphocyte ratio 5
• Fasting lipid profile and blood glucose are standard tests for cardiovascular risk assessment 4

Stroke Management

• For patients with suspected acute or recent TIA or non-disabling ischemic stroke, initial laboratory investigations include:
  • Complete blood count
  • Electrolytes
  • Coagulation studies (aPTT, INR)
  • Renal function (creatinine, e-GFR)
  • Random glucose
  • Troponin 4
• Additional tests may include lipid profile and diabetes screening with HbA1c or oral glucose tolerance test 4

Drug Testing

• Various biological matrices can be used for drug testing: urine, blood, breath, saliva, sweat, and hair 4
• Blood testing is most useful for detecting alcohol and other drug use within 2-12 hours of the test and correlates best with level of impairment 4
• However, blood testing is costly and intrusive, making it less common in primary care settings 4

Alzheimer's Disease and Cognitive Assessment

• Blood-based biomarkers for Alzheimer's disease are emerging but additional data is needed for their use in primary care 4
• Current laboratory tests for cognitive assessment focus on ruling out other causes of cognitive impairment 4

Key Considerations

• The frequency of testing depends on the patient's condition, risk factors, and treatment regimen 4
• Interpretation of test results should consider the patient's clinical context, pre-test probability of disease, and limitations of the tests 2
• Point-of-care testing is becoming increasingly available for various blood tests in primary care settings, allowing for more rapid clinical decision-making 3

Common Pitfalls

• Not considering the prevalence of disease in the population when interpreting test results (lower prevalence leads to lower positive predictive value) 4
• Over-reliance on a single test result without clinical correlation 2
• Failure to repeat abnormal results to confirm findings before extensive workup 2
• Not considering medication effects on laboratory values 1