NBME CBSSA key terms

The proportion of people who truly have a disease and are correctly identified as positive by a test. A highly sensitive test is useful for ruling out disease when negative.

The proportion of people without a disease who are correctly identified as negative by a test. A highly specific test is useful for ruling in disease when positive.

The probability that a person with a positive test result actually has the disease. It rises as disease prevalence in the tested population rises, unlike sensitivity and specificity.

A ratio comparing the probability of a given test result in people with disease versus those without. A positive likelihood ratio above 10 or a negative ratio below 0.1 meaningfully shifts post-test probability.

The proportion of a population that has a disease at a given point in time. It strongly influences predictive values and is distinct from incidence, which counts new cases over time.

The number of new cases of a disease arising in a population over a defined period, divided by the population at risk. It measures how fast new disease develops rather than how much exists.

The ratio of disease incidence in an exposed group to incidence in an unexposed group. It is calculated in cohort studies and directly expresses how much exposure multiplies risk.

The ratio of the odds of exposure among cases to the odds of exposure among controls. It approximates relative risk for rare diseases and is the usual measure in case-control studies.

A range of values within which the true population estimate probably lies, usually stated at 95 percent. If a 95 percent interval for a risk ratio includes 1.0, the result is not statistically significant.

The probability of observing a result at least as extreme as the one found, assuming the null hypothesis is true. It is not the probability that the null hypothesis is correct.

Incorrectly rejecting a true null hypothesis, producing a false-positive conclusion. Its probability is the significance level alpha, conventionally set at 0.05.

Failing to reject a false null hypothesis, producing a false-negative conclusion. Its probability is beta, and statistical power equals 1 minus beta.

The probability that a study will detect a true effect of a given size, equal to 1 minus the type II error rate. Power increases with larger samples, larger effects, and lower variability.

The number of patients who must receive a treatment for one additional patient to benefit, calculated as the reciprocal of the absolute risk reduction. Smaller values indicate greater clinical benefit.

The difference in event rates between a control group and a treated group, expressed as a percentage or proportion. It reflects actual clinical impact better than relative risk reduction.

Apparent improved survival caused only by earlier detection of disease, without any delay in the time of death. Screening studies must account for it when claiming mortality benefit.

The tendency of screening to detect slowly progressive disease disproportionately, making screened patients appear to survive longer. It inflates apparent benefit of screening programs.

A distortion arising when the groups compared differ systematically in ways related to the outcome, often because of how participants were chosen or retained. It threatens the internal validity of a study.

A third variable associated with both exposure and outcome that distorts the observed relationship between them. Randomization, restriction, matching, and stratification are used to control it.

Assigning participants to study groups by chance so that known and unknown confounders are distributed similarly across groups. It is the defining strength of a randomized controlled trial.

Analyzing participants according to the group to which they were originally assigned, regardless of whether they completed the assigned treatment. It preserves randomization but can dilute measured treatment effect.

A retrospective design that identifies people with a disease and comparable people without it, then compares prior exposures. It is efficient for rare diseases but yields odds ratios and is prone to recall bias.

A design that follows groups defined by exposure status forward in time and compares incidence of outcomes. It can establish temporality and yields relative risk, but is inefficient for rare outcomes.

A test applied to asymptomatic people to detect disease early, when treatment may be more effective. It requires high sensitivity and a reasonably high prevalence to be worthwhile.

The physiologic adjustment of carbon dioxide or bicarbonate that partially corrects pH toward normal in a primary acid-base disorder. Respiratory compensation is fast; renal compensation develops over days.

The difference between measured serum sodium and the sum of chloride and bicarbonate, normally about 8 to 12 mEq/L. An elevated gap suggests unmeasured anions such as ketones, lactate, or toxins.

An equation estimating the expected respiratory compensation for metabolic acidosis: expected PCO2 equals 1.5 times bicarbonate plus 8, plus or minus 2. A measured value outside this range signals a second disorder.

The difference between measured serum osmolality and calculated osmolality, normally under 10 mOsm/kg. An elevated gap suggests unmeasured osmoles such as methanol, ethylene glycol, or isopropanol.

The balance of hydrostatic and oncotic pressures across a capillary wall that governs fluid movement between plasma and interstitium. Altering any of these pressures produces edema or volume shifts.

The osmotic pressure exerted by plasma proteins, chiefly albumin, that draws fluid into the vascular space. Low albumin lowers oncotic pressure and promotes interstitial edema.

The fraction of ventricular end-diastolic volume ejected during systole, normally above roughly 55 percent. A reduced value defines systolic heart failure and guides therapy selection.

A hormonal cascade in which renin release leads to angiotensin II production and aldosterone secretion, raising blood pressure and retaining sodium. It is a major target of antihypertensive and heart failure drugs.

Metabolism of an orally absorbed drug by the intestinal wall and liver before it reaches systemic circulation. It reduces oral bioavailability and explains why some drugs require higher oral doses.

The fraction of an administered drug dose that reaches systemic circulation unchanged. Intravenous administration is defined as 100 percent, while oral bioavailability is often lower.

A theoretical volume relating total drug in the body to plasma concentration. Large values indicate extensive tissue uptake, while small values suggest confinement to plasma.

The volume of plasma from which a drug is completely removed per unit time. Clearance determines the maintenance dose needed to sustain a target steady-state concentration.

The time required for plasma drug concentration to fall by half during elimination. It determines dosing interval and the time to reach steady state, generally about four to five half-lives.

The condition in which drug intake equals elimination, so plasma concentration remains stable. It is typically reached after four to five half-lives of regular dosing.

The ratio of the dose producing toxicity to the dose producing the desired effect. A low index means the toxic and therapeutic ranges are close, requiring monitoring of drug levels.