How Doctors Choose the Right Diagnostic Test

Modern medicine has access to an extraordinary array of diagnostic tools. Blood tests, imaging studies, biopsies, genetic tests, functional studies, and various other approaches each provide specific types of information about what is happening inside the body. The combination of these tools can address virtually any clinical question, but the abundance of options creates its own challenge. With so many possible tests available, how do clinicians decide which to order in any given situation? The answer reflects a substantial body of clinical reasoning that combines several specific elements. The clinical question being asked determines what type of information is needed. The differential diagnosis, the list of possible causes for a patient's symptoms, narrows the field of relevant tests. The performance characteristics of each test, including its accuracy and limitations, affect how useful it will be. The patient's specific situation, including risk factors, prior history, and individual considerations, influences the choice. Practical factors including cost, availability, and side effect profiles round out the decision. The judgment that goes into this process is one of the core skills of clinical medicine. Effective diagnosticians do not simply order every test that might be relevant; they select tests that are most likely to provide actionable information for the specific situation. This selectivity matters because tests have downsides as well as benefits. Unnecessary tests can produce false positive results, lead to additional unnecessary investigations, expose patients to risks, and consume resources without producing benefits. The thoughtful selection of testing is part of good medical practice rather than an obstacle to thorough evaluation. This article explains the reasoning that goes into diagnostic test selection, what factors clinicians consider, how patients can engage meaningfully with these decisions, and what to do when concerns arise about whether the appropriate testing has been ordered. For more on diagnostic imaging specifically, see Advanced Imaging Technologies in Medicine.

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The Goal of Diagnostic Testing

Before discussing test selection specifically, understanding what diagnostic tests are designed to accomplish helps frame the broader question.

Core Purposes of Diagnostic Testing

Diagnostic tests serve several specific purposes:

• Confirming or excluding suspected conditions when the clinical picture suggests a particular diagnosis
• Identifying the cause of symptoms when multiple conditions could be responsible
• Characterizing the extent or stage of a known condition to guide treatment
• Monitoring response to treatment over time
• Screening for conditions in asymptomatic individuals at elevated risk

Each of these purposes has somewhat different implications for test selection. Confirming a strongly suspected diagnosis often calls for tests with high specificity that minimize false positives. Excluding a condition often calls for tests with high sensitivity that minimize false negatives. Screening tests in healthy individuals must balance the benefits of early detection against the risks of false positives in the general population.

The specific purpose of testing in any given situation shapes the appropriate choice of tests.

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The Clinical Question Drives the Selection

The most fundamental factor in diagnostic test selection is what clinical question is being asked.

Example: Chest Pain

A patient with chest pain might have many possible underlying causes. Each possible cause has different appropriate tests:

• Cardiac causes might warrant electrocardiogram, troponin testing, and possibly stress testing or coronary angiography
• Pulmonary causes might warrant chest X-ray, CT scan, or specific tests for pulmonary embolism
• Gastrointestinal causes might warrant different tests entirely

The specific clinical features narrow the differential and direct the testing.

Example: Abdominal Pain

A patient with abdominal pain similarly has many possible causes that direct different tests:

• Right upper quadrant pain often warrants ultrasound for evaluation of the gallbladder
• Right lower quadrant pain in young adults often warrants CT scan for evaluation of appendicitis
• Pelvic pain in women often warrants ultrasound for gynecological evaluation

The clinical question, refined through history and examination, determines which tests are most relevant. The skilled clinician asks specific questions in the history and performs specific examination maneuvers to refine the differential, then selects tests based on this refined question rather than ordering everything that could potentially be relevant.

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The Differential Diagnosis

The differential diagnosis is the list of possible conditions that could explain the patient's symptoms. Generating an appropriate differential is a core skill of clinical reasoning.

Building the Differential

The differential typically begins broad and is narrowed through history, examination, and initial testing:

• Most common conditions are usually considered first
• Less common conditions are added based on specific features
• Life-threatening conditions warrant attention even when less likely, since missing them produces serious consequences

Using the Differential to Select Tests

Once the differential is established, tests are selected to distinguish among the possibilities. The ideal tests are those that produce different results for different conditions on the differential, allowing efficient narrowing of the possibilities.

For some clinical situations, a few specific tests address most of the differential. For others, multiple tests with different specific purposes contribute to the diagnostic picture.

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Pretest Probability

A central concept in diagnostic test selection is pretest probability, the likelihood that a patient has a particular condition before any specific test is performed. The pretest probability substantially influences how to interpret test results and which tests are most useful.

High Pretest Probability

For a condition with high pretest probability based on the clinical picture:

• Positive test confirms the diagnosis with high confidence
• Negative test may not necessarily exclude the diagnosis, particularly if the test is imperfect

Low Pretest Probability

For a condition with low pretest probability:

• Negative test confirms its absence with high confidence
• Positive test may represent a false positive rather than a true diagnosis, particularly in low-prevalence situations

Implications for Test Selection

The implication for test selection is that the same test may be more or less useful depending on the clinical situation. A test that is excellent for confirming a strongly suspected diagnosis may be misleading when used to screen for the same condition in a population where it is rare.

The skilled clinician estimates pretest probability based on the clinical picture and selects tests whose information will substantially modify this probability toward a clearer diagnosis.

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Test Performance Characteristics

Diagnostic tests vary in their accuracy. Several specific characteristics describe how well a test performs.

Sensitivity

Sensitivity is the proportion of patients with a condition who have a positive test. A highly sensitive test rarely misses the condition when it is present. High sensitivity is particularly important for screening tests, where missing the condition could have serious consequences.

A test with 95 percent sensitivity for a condition produces positive results in 95 percent of patients with the condition, and negative (false negative) results in 5 percent.

Specificity

Specificity is the proportion of patients without the condition who have a negative test. A highly specific test rarely produces positive results in patients without the condition. High specificity is particularly important when a positive result triggers significant intervention, since false positives lead to unnecessary procedures.

A test with 95 percent specificity produces negative results in 95 percent of patients without the condition, and positive (false positive) results in 5 percent.

Positive and Negative Predictive Values

Beyond sensitivity and specificity, predictive values describe how to interpret a specific test result:

• Positive predictive value is the proportion of patients with a positive test who actually have the condition
• Negative predictive value is the proportion of patients with a negative test who actually do not have the condition

These values depend not only on the test's performance characteristics but also on the prevalence of the condition in the population being tested. For rare conditions, even highly specific tests can produce many false positives in absolute numbers because of the larger pool of unaffected individuals being tested.

Practical Implications

The implications for test selection include several specific points:

• High sensitivity tests are good for ruling out conditions when negative
• High specificity tests are good for ruling in conditions when positive
• Most useful test for a specific situation depends on whether ruling in or ruling out is the primary goal

For screening, sensitivity often matters most, with confirmatory testing for positive results addressing specificity concerns. For confirming a suspected diagnosis, specificity often matters more.

For more on how imaging interpretation works, see Can Medical Imaging Be Misinterpreted?.

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Patient-Specific Factors

Beyond the clinical question and test characteristics, patient-specific factors influence test selection.

Age

Age affects test choice in multiple ways:

• Some tests are more or less appropriate at different ages
• Pretest probability for many conditions varies with age
• Side effects and contraindications of certain tests vary with age

For children, considerations include developmental impacts of radiation, the difficulty of certain procedures, and the need for sedation. For older adults, considerations include comorbidities, kidney function affecting contrast administration, and various other factors.

Pregnancy

Pregnancy substantially affects test selection:

• Ionizing radiation tests are generally avoided when alternatives exist
• Specific medications or contrast agents may have particular considerations
• Clinical question may need to be addressed through different testing approaches than would be used in non-pregnant patients

Comorbidities

Existing medical conditions affect test selection:

• Kidney function affects the safety of contrast-based imaging
• Cardiac conditions affect the safety of certain stress tests
• Various other conditions create specific contraindications or precautions for different tests

Allergies

Allergies, particularly to contrast media or specific medications, affect test selection:

• Iodine-based contrast for CT scans
• Gadolinium-based contrast for MRI
• Various other materials have potential for reactions in susceptible patients

Risk Factor Profile

The patient's overall risk factor profile affects pretest probability for many conditions:

• Smokers have different pretest probabilities for many conditions than non-smokers
• Patients with strong family histories have different probabilities than those without
• Various other risk factors similarly affect the diagnostic picture

Prior Test Results

Prior testing affects the choice of additional tests:

• Tests already performed may not need repeating
• Equivocal results from prior testing may direct different next steps than would be chosen without that prior information

Patient Preferences

Patient preferences for specific tests, when relevant alternatives exist, can affect test selection. Some patients have strong preferences about radiation exposure, invasive procedures, or various other aspects of testing.

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Practical Considerations

Beyond the clinical and patient factors, practical considerations affect test selection.

Availability

Tests vary in their availability:

• Some tests are available in most clinical settings
• Others require specialized facilities or expertise
• Local availability of a test affects whether it can be ordered easily or requires referral and travel

Cost

Cost is a meaningful consideration for many patients:

• Insurance coverage varies among tests
• Cost-benefit consideration of any specific test affects whether it should be ordered
• Alternatives may provide similar information at lower cost

Time Required

Time considerations affect test selection:

• Some tests provide immediate results
• Others require days or weeks
• Urgent situations favor faster tests
• Non-urgent evaluations may accept slower but more comprehensive tests

Side Effects and Risks

The side effects and risks of testing affect the appropriate choice:

• Significant risks are reserved for situations where potential benefits clearly justify the risks
• Less invasive alternatives are preferred when they can answer the clinical question

Discomfort

Patient comfort during testing affects choice when alternatives exist. Tests requiring sedation, prolonged immobility, or specific positioning have practical considerations that may favor alternative approaches.

For more on imaging safety considerations, see Are Medical Imaging Tests Safe? Understanding Radiation Risks.

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Common Categories of Diagnostic Tests

Diagnostic tests span many categories, each with specific applications.

Blood Tests

Blood tests are among the most commonly used diagnostic tools. They can evaluate:

• General health markers (complete blood count, basic metabolic panel)
• Specific organ function (liver function tests, kidney function tests)
• Specific disease markers (cardiac markers, tumor markers)
• Hormonal status (thyroid function, reproductive hormones)
• Inflammatory markers (CRP, ESR)
• Various specific tests for particular conditions

Blood tests are generally accessible, relatively inexpensive, and provide rapid results. They are often the first-line testing for many situations.

Urine Tests

Urine tests can:

• Evaluate kidney function
• Identify infections
• Detect various metabolic conditions
• Screen for certain other diseases

Urine tests are simple to perform and provide useful information for many clinical situations.

Imaging Studies

Imaging studies allow visualization of internal structures. Different modalities have different applications:

• X-rays provide rapid evaluation of bones and certain other structures
• CT scans provide detailed cross-sectional images
• MRI provides detailed soft tissue evaluation
• Ultrasound provides real-time imaging without radiation
• Nuclear medicine studies evaluate functional and metabolic aspects

The choice among imaging modalities depends on the specific clinical question and patient factors. For more on this, see Ultrasound vs MRI: When Each Is Used in Diagnosis.

Biopsies

Biopsies provide tissue diagnosis when imaging or other approaches cannot definitively characterize a finding. The procedure involves removing a tissue sample for examination by a pathologist. Different biopsy techniques apply to different situations.

Functional Studies

Functional studies evaluate how organs and systems work, not just their structure. Examples include:

• Cardiac stress tests
• Pulmonary function tests
• Electroencephalogram
• Various others

These tests provide information about function that anatomic imaging cannot provide.

Genetic and Molecular Tests

Genetic and molecular tests have expanded substantially over recent decades. They can:

• Identify specific mutations associated with disease
• Characterize tumors for targeted therapy decisions
• Evaluate inherited conditions
• Various other applications

These tests have become increasingly central to many areas of medicine.

Endoscopic Procedures

Endoscopic procedures use cameras to directly visualize internal structures. Common examples include:

• Colonoscopy for evaluation of the large intestine
• Upper endoscopy for evaluation of the upper digestive tract
• Bronchoscopy for evaluation of the airways
• Various others

These procedures can also obtain tissue samples for biopsy.

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When Multiple Tests Are Needed

For many clinical situations, no single test answers all relevant questions. Multiple tests work together to provide comprehensive evaluation.

Test Sequencing

The sequence of testing matters:

• Initial tests typically address the most likely diagnoses
• Initial tests provide information that guides subsequent testing
• Later tests address remaining questions
• Later tests provide more detailed information about findings identified earlier

Each test should typically have a specific purpose in the diagnostic strategy. Tests ordered without clear purpose may produce results that complicate rather than clarify the picture.

Complementary Information

The combination of tests should provide complementary information rather than redundant information. Tests that provide similar information add cost without adding meaningful value.

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When Tests Produce Unexpected Results

Test results sometimes do not match the clinical expectation. Several considerations apply.

False Positive Results

False positive results occur when a test indicates the presence of a condition that is not actually present. The probability of false positives is higher in low-prevalence situations. False positives often require additional testing to clarify.

False Negative Results

False negative results occur when a test misses a condition that is actually present. False negatives can lead to delayed diagnosis if not recognized. When clinical suspicion remains high despite a negative test, additional evaluation is often warranted.

Incidental Findings

Incidental findings are findings unrelated to the original clinical question that are identified during testing. CT scans of the chest sometimes find incidental thyroid nodules. Ultrasounds of the abdomen sometimes find incidental kidney findings. Various other incidental findings emerge from imaging.

These findings require judgment about whether they warrant additional evaluation or can be safely ignored. The decision often depends on the specific findings and patient factors.

Results That Do Not Match Clinical Picture

When results do not match the clinical picture, several considerations apply:

• The test may have produced a false result
• The condition may be unusual in some way that does not fit standard patterns
• Additional testing may be needed to resolve the discrepancy
• Sometimes the clinical picture warrants re-evaluation in light of the test results

For more on this situation, see Can Medical Imaging Be Misinterpreted?.

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Avoiding Unnecessary Testing

The selectivity of test ordering is part of good medical practice. Several specific reasons support avoiding unnecessary testing.

False Positive Cascades

Testing in situations where pretest probability is low frequently produces false positive results. Each false positive may trigger additional testing, sometimes with its own risks. The cascade of investigations triggered by an initial false positive can produce substantial harm without ultimately revealing actual disease.

Costs

Tests have costs that affect both individual patients and the broader healthcare system. Unnecessary tests consume resources that could be applied elsewhere.

Side Effects and Risks

Tests have side effects and risks:

• Simple blood tests carry small risks of bruising, infection, or other complications
• More invasive tests carry more substantial risks
• Avoiding unnecessary tests avoids unnecessary exposure to these risks

Anxiety

Testing can produce anxiety for patients:

• Waiting for results
• Dealing with equivocal findings
• Pursuing additional investigations

All have psychological costs. Tests that are unlikely to produce useful information may not be worth these costs.

Practical Burden

Testing imposes practical burdens including:

• Time requirements
• Scheduling challenges
• Transportation needs
• Various others

For elderly patients, those with limited resources, and various other groups, the practical burden of testing can be substantial.

Test Stewardship

The concept of test stewardship has emerged as a recognition that thoughtful selection of testing is part of good practice. Initiatives like Choosing Wisely have identified specific tests that are commonly overordered and recommended more selective approaches.

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When Patients Have Concerns About Testing

Several situations may produce patient concerns about testing decisions.

Concerns That a Test Has Not Been Ordered

When a patient feels that a specific test should have been ordered but has not been, several approaches help address the concern:

• Discussion with the physician about the reasoning behind the testing decisions provides clarity
• Asking specifically about the differential diagnosis being considered helps understand the diagnostic strategy
• Asking about alternative approaches that could provide similar information may identify what tests are being substituted for those the patient expected

Concerns That Too Many Tests Are Being Ordered

When a patient feels that too many tests are being ordered, similar discussion can help:

• Asking about the specific purpose of each test
• Asking about alternatives being considered
• Understanding the contribution each test makes to the overall picture

Disagreement About Testing

When there is genuine disagreement about appropriate testing, second opinions provide additional perspectives. Different physicians may have different judgments about appropriate testing in any given situation, and the diversity of opinions can help patients understand the range of reasonable approaches.

For more on second opinions, see What Is a Medical Second Opinion and Why It Matters.

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Shared Decision Making

Modern medical practice increasingly emphasizes shared decision making, where patients and physicians together decide about testing and other care. The approach acknowledges that medical decisions often involve trade-offs that depend on individual values and preferences.

Elements of Shared Decision Making

For testing decisions, shared decision making involves discussion of:

• What is known about the patient's situation
• What tests could potentially provide useful information
• What strengths, limitations, and risks each test has
• What the patient's preferences and priorities are
• What testing strategy makes most sense given all these factors

The discussion does not require equal medical knowledge between physician and patient. The patient brings knowledge of their own values, preferences, and individual circumstances. The physician brings medical expertise about diagnostic options. Together, they can make decisions that integrate both types of knowledge.

For patients who want more active engagement in testing decisions, communicating this preference to the physician supports more collaborative decision making.

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Different Practice Patterns

Different physicians may approach testing decisions differently. The variations reflect legitimate differences in clinical judgment as well as differences in training, practice settings, and various other factors.

Variations in Testing

Some physicians order more testing than others for similar clinical situations. The variation often reflects:

• Different judgments about pretest probability
• Different thresholds for action
• Different approaches to managing uncertainty

Implications for Patients

The variation among physicians has implications for patients:

• Testing approach with one physician may differ from that with another
• For patients seeing multiple physicians or seeking second opinions, encountering different testing approaches is common

Neither Approach Is Universally Better

Neither more nor less testing is universally better. The appropriate level of testing depends on the specific clinical situation, balancing the benefits of additional information against the costs and risks. Different reasonable physicians may strike this balance differently.

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Special Situations

Several specific situations have particular considerations regarding testing decisions.

Emergency Situations

Emergency situations often require rapid testing decisions to address potentially life-threatening conditions:

• Threshold for testing may be lower because of the consequences of missing serious diagnoses
• Available time for evaluation is limited, requiring efficient selection of tests

Chronic Conditions

For chronic conditions, testing strategies often emphasize monitoring over time rather than initial diagnosis:

• Periodic testing tracks disease progression and treatment response
• Patterns of testing differ from those used for initial evaluation of acute symptoms

Cancer Diagnosis and Staging

Cancer diagnosis and staging involves comprehensive testing strategies:

• Initial diagnosis often requires biopsy
• Staging requires multiple tests to characterize the extent of disease
• Treatment monitoring continues with periodic testing throughout care

Preventive Care

Preventive care involves screening tests in asymptomatic individuals. The selection of screening tests depends on:

• Age
• Sex
• Family history
• Various other risk factors

Screening recommendations come from professional organizations based on evidence about benefits and harms in specific populations.

Genetic Testing

Genetic testing has specific considerations including:

• Implications for family members
• Potential for results that have uncertain meaning
• Psychological impact of genetic findings

Genetic counseling often supports decisions about genetic testing.

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Common Misconceptions

Several misconceptions about diagnostic testing deserve attention.

More Testing Is Always Better

Misleading. Unnecessary testing can produce false positives, additional unnecessary investigations, and various other harms.

Negative Test Definitively Excludes Condition

Overstates certainty. Tests are imperfect, and false negatives occur. When clinical suspicion remains high despite negative testing, additional evaluation may be warranted.

Positive Test Definitively Confirms Condition

Similarly overstates certainty. False positives occur, particularly in situations of low pretest probability. Confirmatory testing may be needed.

Same Test Should Be Ordered in Similar Situations

Overlooks the importance of patient-specific factors. The right test depends on the individual situation, not just the clinical category.

Doctors Who Order More Tests Are More Thorough

Not accurate. Thoughtful selection of tests reflects better clinical judgment than indiscriminate testing.

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When to See a Specialist

Several situations warrant specialist consultation regarding testing decisions.

• When primary care testing has not resolved the clinical question, specialty consultation can provide additional expertise about appropriate next testing
• When complex conditions require specialized expertise, specialty evaluation includes specialty-specific testing approaches
• When patients have concerns about testing decisions that have not been resolved through discussion with the primary physician, second opinions can provide additional perspectives
• When test results are unusual or difficult to interpret, specialty consultation often provides clarification

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Conclusion

The selection of diagnostic tests involves substantial clinical reasoning combining the clinical question, the differential diagnosis, the performance characteristics of available tests, patient-specific factors, and practical considerations. Effective diagnosticians select tests that are likely to provide actionable information for the specific situation, rather than ordering everything that could potentially be relevant.

For patients, understanding how testing decisions are made supports informed engagement with care. The reasoning behind specific test choices is generally explainable when asked. The variation among physicians in testing approaches reflects legitimate differences in clinical judgment rather than universal right or wrong answers.

The encouraging element of modern diagnostics is that the available tools provide unprecedented capability to understand what is happening inside the body. The combination of blood tests, imaging studies, biopsies, genetic tests, and various other approaches can address virtually any clinical question. The challenge is selecting the right tests for each specific situation, balancing the benefits of additional information against the costs and risks of testing.

For most patients, the practical message is that testing decisions reflect clinical judgment about what will be most useful for their specific situation. When questions arise about testing, discussion with the physician typically clarifies the reasoning. When concerns persist, second opinions can provide additional perspectives. The trajectory of better diagnostic care comes from thoughtful selection of testing rather than from maximizing the number of tests ordered.

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Medical Disclaimer

This content is for informational purposes only and does not constitute medical advice. Individuals should consult a qualified healthcare provider for diagnosis and treatment.