Screening Fundamentals - The Why & How
- Sensitivity: Detects disease in those who have it. $Sn = \frac{TP}{TP+FN}$
- Specificity: Rules out disease in those who don't. $Sp = \frac{TN}{TN+FP}$
- Positive Predictive Value (PPV): Probability of disease if test is positive. $PPV = \frac{TP}{TP+FP}$
- Negative Predictive Value (NPV): Probability of no disease if test is negative. $NPV = \frac{TN}{TN+FN}$
⭐ PPV and NPV are heavily influenced by disease prevalence. ↑ Prevalence → ↑ PPV & ↓ NPV.
- Wilson-Jungner Criteria: An effective screening program requires:
- Important problem, understood natural history.
- Accepted treatment, available facilities.
- Suitable & acceptable test.
- Cost-effective & continuous process.
📌 SPIN & SNOUT: SPecific test, when Positive, rules IN disease. SNensitive test, when Negative, rules OUT disease.
USPSTF Grades - Letters of the Law
The U.S. Preventive Services Task Force (USPSTF) uses a grading system to categorize the strength of evidence and magnitude of benefit for preventive services.
| Grade | Recommendation | Clinical Action |
|---|---|---|
| A | Recommended. High certainty of substantial net benefit. | Offer or provide this service. |
| B | Recommended. High certainty of moderate net benefit or moderate certainty of moderate to substantial net benefit. | Offer or provide this service. |
| C | Selective Recommendation. Offer for individual patients based on professional judgment and patient preferences. At least moderate certainty of small net benefit. | Offer or provide for selected patients. Consider individual circumstances. |
| D | Not Recommended. Moderate or high certainty of no net benefit or that harms outweigh benefits. | Discourage the use of this service. |
| I | Insufficient Evidence. Evidence is lacking, of poor quality, or conflicting. Benefit cannot be determined. | Read the clinical considerations. If the service is offered, patients should understand the uncertainty. |
⭐ Grade C recommendations are the most nuanced. The decision to screen rests on a collaborative conversation between the clinician and the patient, weighing the small potential benefit against individual patient factors like risk profile and personal values.
Special Populations - High-Risk Nuances
Screening protocols adapt for individuals with higher baseline risk due to genetics, conditions, or exposures. The goal is earlier detection through more frequent or specialized testing.
| Special Population | Condition | Screening Guideline (vs. General Population) |
|---|---|---|
| Pregnancy | Asymptomatic Bacteriuria | Urine culture at first prenatal visit (12-16 wks). Not routinely screened. |
| Gestational Diabetes | 50g 1-hr glucose challenge at 24-28 wks. Not screened unless risk factors exist. | |
| Group B Strep | Rectovaginal culture at 36-37 wks. Not screened. | |
| Strong Family Hx | BRCA1/2 Mutation | Annual Breast MRI starting age 25; annual mammogram at 30. (vs. mammogram at 40-50). |
| Lynch (HNPCC) | Colonoscopy at 20-25, repeat q1-2 yrs. (vs. age 45, q10 yrs). | |
| Immunocompromised | HIV & Cervical Cancer | Pap smear at diagnosis, then annually. (vs. q3 yrs). |
| HIV (MSM) | Annual anal Pap test for Human Papillomavirus (HPV) related dysplasia. Not screened. |
High-Yield Points - ⚡ Biggest Takeaways
- Smokers with a ≥20-pack-year history require annual low-dose CT for lung cancer screening from age 50-80.
- Patients with cirrhosis need biannual liver ultrasound (+/- AFP) to screen for hepatocellular carcinoma.
- A first-degree relative with CRC <60 prompts colonoscopy at age 40 or 10 years before the relative's diagnosis.
- BRCA carriers undergo intensive screening with annual mammograms and breast MRIs.
- Lynch syndrome (HNPCC) requires colonoscopy every 1-2 years beginning at age 20-25.
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