Diagnostic test (medical)

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A diagnostic test is, as its name implies, a medical test or series of tests designed to examine a patient's signs or symptoms (what hurts, or what otherwise seems abnormal to the patient) in order to allow a medical practitioner to give a diagnosis (a conclusion) about what is wrong drawn an analysis of the patient's test results. This is the first step in deciding how to treat the ailment or disease.

Some diagnostic tests may be similar to screening tests, however they differ from the latter in that screening tests are designed to discover abnormality before any symptoms are manifested; diagnostic tests take place after the patient has notice symptoms of abnormality, illness or disease.

Interpreting diagnostic tests

See Sensitivity and specificity and Bayes Theorem

Non-specific benefit of tests

Medical tests can have value when results are abnormal by explaining to a patient the cause of their symptoms[1]. In addition, normal test results can have value by reassuring patients that serious illness is not present and even reduce the rates of subsequent symptoms [2].

If a normal test result is expected, understanding the meaning of a normal test in advance of learning the test results may reduce the rates of subsequent symptoms.[3][4]

Non-specific harm of tests

Lack of adequate education about the meaning of test results (especially relevant to tests that may have incidental and unimportant findings) may cause an increase in symptoms[5] or anxiety[6]. This may be similar to the effects of labeling.[7]

In addition, the possible benefits must be weighed against the costs of unnecessary tests and resulting unnecessary follow-up and possibly even unnecessary treatment of incidental findings.[8]

Tests that seem harmless individually, may be harmful when repeated multiple times in a patient. For example in radiology, it is estimated that computed tomography may be contributing to cancer.[9]

Strategies to reduce unnecessary diagnostic testing

Improve availability of prior results

Sometimes testing is redundant.[10] Having the results of prior tests available may reduce the need for repeating tests.[11] A randomized controlled trial has shown reduction i ordering of redundant tests.[12]

Delay testing

Randomized controlled trials show benefit of immediate versus delayed testing in patients without possible emergent conditions.[5][8] The benefit may be in part due to successful empirical treatment.

Establish an alternative diagnoses

Studies show that the chance of thromboembolism is less in patients who have have alternative explanations for their symptoms.

Patients with chronic abdominal symptoms are less likely to have underlying organic disease if they meet criteria for irritable bowel.

Patients with new headaches are less likely to have significant underlying pathology if their headaches meet criteria for being a migraine headache.[13]

Among patients referred for endoscopy, psychiatric diagnoses are associated with normal endoscopies.[14]

Recognize futility of testing when disease prevalence is extremely low

Using Bayes Theorem may allow recognition that there are some settings where testing can be considred futile. Two conditions are necessary to establish futility:

  1. Being able to estimate the post-test probability of disease by having all the nessary information to do this: sensitivity and specificity and prevalence of disease.
  2. Evidence-based analysis of what post-test probability of disease is considered futile. For example, in the screening of HIV, decision analysis calculates that screening should occur whenever prevalence is approximately 0.2%.[15] However, this type of analysis is not available for many diseases and, when is available, usually includes value judgments about futility and cost that may not be universally accepted judgments.

Examples where thresholds are established or implied to justify testing include:

  • HIV screening - the threshold is very low
  • New seizure - the threshold is very low
  • Headaches - if the headache is chronic and stable
  • Pharyngitis - the threshold is higher as the stakes are lower
  • Influenza - the threshold is higher as the stakes are lower

In the absence of specific analysis, another approach to determining the appropriate threshold is to use precedent. For example, in potentially lethal diseases such as pulmonary embolism[16], acute coronary syndrome[17], and pneumonia[18][19], in the best of health care settings 2-4% of patients have their diagnosis missed.Therefore, the precedent would be that whenever a serious disease is estimated to have more than a 2%-4% prevalence, the disease should be sought.

A randomized controlled trial showed a small reduction in test ordering when a computer displayed very low probabilities that a test would be abnormal.[20]


References

  1. ↑ Ward B, Wu W, Richter J, Hackshaw B, Castell D (1987). "Long-term follow-up of symptomatic status of patients with noncardiac chest pain: is diagnosis of esophageal etiology helpful?". Am J Gastroenterol 82 (3): 215-8. PMID 3826028.
  2. ↑ Sox H, Margulies I, Sox C (1981). "Psychologically mediated effects of diagnostic tests". Ann Intern Med 95 (6): 680-5. PMID 7305144.
  3. ↑ Petrie K, Müller J, Schirmbeck F, Donkin L, Broadbent E, Ellis C, Gamble G, Rief W (2007). "Effect of providing information about normal test results on patients' reassurance: randomised controlled trial". BMJ 334: 352. PMID 17259186.
  4. ↑ Thomas Mordekhai Laurence (2004). Extreme Clinic -- An Outpatient Doctor's Guide to the Perfect 7 Minute Visit. Philadelphia: Hanley & Belfus. ISBN 1-56053-603-9. 
  5. ↑ 5.0 5.1 Kendrick D, Fielding K, Bentley E, Kerslake R, Miller P, Pringle M (2001). "Radiography of the lumbar spine in primary care patients with low back pain: randomised controlled trial". BMJ 322 (7283): 400-5. PMID 11179160.
  6. ↑ Hoefman E, Boer KR, van Weert HC, Reitsma JB, Koster RW, Bindels PJ (2007). "Continuous event recorders did not affect anxiety or quality of life in patients with palpitations". Journal of clinical epidemiology 60 (10): 1060–6. DOI:10.1016/j.jclinepi.2007.01.014. PMID 17884602. Research Blogging.
  7. ↑ Haynes RB, Sackett DL, Taylor DW, Gibson ES, Johnson AL (1978). "Increased absenteeism from work after detection and labeling of hypertensive patients". N. Engl. J. Med. 299 (14): 741–4. PMID 692548. [e]
  8. ↑ 8.0 8.1 Jarvik J, Hollingworth W, Martin B, Emerson S, Gray D, Overman S, Robinson D, Staiger T, Wessbecher F, Sullivan S, Kreuter W, Deyo R (2003). "Rapid magnetic resonance imaging vs radiographs for patients with low back pain: a randomized controlled trial". JAMA 289 (21): 2810-8. PMID 12783911.
  9. ↑ Brenner DJ, Hall EJ (2007). "Computed tomography--an increasing source of radiation exposure". N. Engl. J. Med. 357 (22): 2277–84. DOI:10.1056/NEJMra072149. PMID 18046031. Research Blogging.
  10. ↑ Bates DW, Boyle DL, Rittenberg E, et al (1998). "What proportion of common diagnostic tests appear redundant?". Am. J. Med. 104 (4): 361–8. PMID 9576410. [e]
  11. ↑ Tierney WM, McDonald CJ, Martin DK, Rogers MP (1987). "Computerized display of past test results. Effect on outpatient testing". Ann. Intern. Med. 107 (4): 569–74. PMID 3631792. [e]
  12. ↑ Bates DW, Kuperman GJ, Rittenberg E, et al (1999). "A randomized trial of a computer-based intervention to reduce utilization of redundant laboratory tests". Am. J. Med. 106 (2): 144–50. PMID 10230742. [e]
  13. ↑ Detsky ME, McDonald DR, Baerlocher MO, Tomlinson GA, McCrory DC, Booth CM (2006). "Does this patient with headache have a migraine or need neuroimaging?". JAMA 296 (10): 1274–83. DOI:10.1001/jama.296.10.1274. PMID 16968852. Research Blogging.
  14. ↑ O'Malley PG, Wong PW, Kroenke K, Roy MJ, Wong RK (1998). "The value of screening for psychiatric disorders prior to upper endoscopy". Journal of psychosomatic research 44 (2): 279–87. PMID 9532557. [e]
  15. ↑ Paltiel AD, Walensky RP, Schackman BR, et al (2006). "Expanded HIV screening in the United States: effect on clinical outcomes, HIV transmission, and costs". Ann. Intern. Med. 145 (11): 797–806. PMID 17146064. [e]
  16. ↑ Stein PD, Fowler SE, Goodman LR, et al (2006). "Multidetector computed tomography for acute pulmonary embolism". N. Engl. J. Med. 354 (22): 2317–27. DOI:10.1056/NEJMoa052367. PMID 16738268. Research Blogging.
  17. ↑ Selker HP, Beshansky JR, Griffith JL, et al (1998). "Use of the acute cardiac ischemia time-insensitive predictive instrument (ACI-TIPI) to assist with triage of patients with chest pain or other symptoms suggestive of acute cardiac ischemia. A multicenter, controlled clinical trial". Ann. Intern. Med. 129 (11): 845–55. PMID 9867725. [e]
  18. ↑ Fine MJ, Auble TE, Yealy DM, et al (1997). "A prediction rule to identify low-risk patients with community-acquired pneumonia". N. Engl. J. Med. 336 (4): 243–50. PMID 8995086. [e]
  19. ↑ Carratalà J, Fernández-Sabé N, Ortega L, et al (2005). "Outpatient care compared with hospitalization for community-acquired pneumonia: a randomized trial in low-risk patients". Ann. Intern. Med. 142 (3): 165–72. PMID 15684204. [e]
  20. ↑ Tierney WM, McDonald CJ, Hui SL, Martin DK (1988). "Computer predictions of abnormal test results. Effects on outpatient testing". JAMA 259 (8): 1194–8. PMID 3339821. [e]
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