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Interpreting Laboratory Tests

Interpreting Laboratory Tests. Clinicians in all areas of medical practice are dependent on lab. testing to arrive at a correct diagnosis. Patients tell doctors whether they are healthy or not. Lab. Tests somehow have not magic power by themselves to detect disease.

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Interpreting Laboratory Tests

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  1. Interpreting Laboratory Tests

  2. Clinicians in all areas of medical practice are dependenton lab. testing to arrive at a correct diagnosis. • Patients tell doctors whether they are healthy or not. Lab. Tests somehow have not magic power by themselves to detect disease. • There is the same uncertainty in a test result as there is in a philosophic's determination to get a handle on what is the truth. • To better understand what a test result means the clinicians should be aware of the science behind the test.

  3. The Concept of Normal • Clinical decision making by lab. testing is based on the assumption that a given result can divide patient into two groups diseased or disease free (which has several limitations). Under ideal circumstances the test should be accurate and precise. Accuracy is the ability of the test measurement to reflect the true value. However our best estimate of truth is governed by comparison against a gold standard which itself is not perfect.

  4. Precision is a measure of the reproducibility of a test measurement when the same specimen is rechecked under the same circumstances with freedom from bias. Repeated measurements will always have variability but to be precise the range of variability needs to substantially less than the normal range for that test. • The normal range is determined by testing a group of indiv. Who are supposedly disease free and demographically representative of indiv. who could potentially be affected by disease in question.

  5. The normal range will include 95% of indiv. tested, 5% of population tested will fall outside the normal range but be disease free. Ordering of multiple tests further complicate the concept of normal. Before ordering a test decide what you will do if it is positive or negative and if both answer are the same do not do the test. • Have a specific reason for ordering a test rather than a shotgun testing approach in which an apparently healthy person has a bettery of tests performed just to see what turns up.

  6. Nondisease factors that may cause lab. values to fall outside the normal range • Pregnancy, Athletics, Neonatal period, Posture, Food intake, Prescribing drugs, O.C.P, Ethanol, Haemolysis or clotting specimen, Diurnal variation, Age, Gender, race, Coexisting disease, etc.

  7. Multiple tests ordering and the probability that a healthy person will be labeled as abnormal • No. of independent tests Probability of an abnormal result (%) 1 5 2 10 5 23 10 40 20 64 50 92 90 99 Infinity 100

  8. Evaluate a Test's Performance Characteristics Given that tests are not totally accurate or precise one must have a way to quantify the short comings. A test's ability to discriminate diseased from non disease indiv. is defined by its sensitivity, specificity and positive and negative predictive values.

  9. Diagnostic Test Performance Characteristics

  10. Sensitivity = TP__ TP + FN • Specificity = __TN___ TN + FP • Positive Predictive Value = ___TP__ TP + FP • Negative Predictive Value = ___TN____ TN + FN

  11. Sensitivity= Percentage of persons with the disease who are correctly identified by the test. • Specificity= Percentage of persons who are disease free who are correctly excluded by the test. • Positive predictive value is the percentage of persons with a positive test who actually have the disease. • Negative predictive value is the percentage of persons with a negative test who do not have the disease.

  12. Medically Acceptable factors used to determine the Balance between sensitivity and specificity ↑ Sensitivity & ↓ Specificity↓ Sensitivity & ↑ Specificity Testing is cheap Testing is expensive Testing is easy to administer Testing is technically difficult to perform Testing is readily available Testing is rational Testing is safe Testing has sign. side effects Treatment is safer Treatment has toxic or dangerous side effect Treatment is effective Treatment has low effectiveness Treatment is inexpensive Treatment is costly Undetected disease has serious physical, Falsely labeling people on having a disease Psychological consequences has serious repercussions Compelling reason to confirm disease Compelling reasons to exclude disease Follow up care as mean to Dx is difficult Follow up care is easy

  13. If the pretest probability of disease is low based on the history and physical examination do not test and do not treat for the suspected disease. 2. If the pretest probability is high treat without testing because the person probably already has the disease and the test does not add much. 3. If the pretest probability is in the middle then test and treat only those with a positive result.

  14. Potassium • High conc. Intracellular. Hyperkalamin is defined as serum K > 5.1 mmol/L Occasionally Hyperkalaemia can be artificial (pseudohyperkalaemia) caused by thrombocytosis, leucocytosis or haemolysis. • Intracellular shift with ↑ cellular K release occurs in acute metabolic acidosis, crush injury, burns, insulin deficiency, betablockade & haemolysis. • K exertion by kidney is flow dependent so hyperkalaemia usually develop only in low flow states or CRF with GFR less than 5mls/min. therefore oligurea & anuria are imp. causes of hyperkalaemia.

  15. Hyperkalaemia can be caused by drugs , ACEI, Spironolactone, Triamterine, Amiloride, NSAIDS, Heparin cyclosporine, Pentamidine. • Hypokalamia is associated with serum K level of less than 3.5mm.l/L. During evaluation of hypokalaem mg should be checked because hypomagnesemia is a cause of hypokalaemia.

  16. Pregnancy Test • Current serum assay can detect pregnancy approx. 8-10 days after conception. In first 4-8 weeks of pregnancy hCG level double approx. every 2 days (failure of doubling suggest Ectopic pregnancy or abortion). • For 1st 2 weeks after conception serum level of hCG are higher than urine level but after 3 weeks of pregnancy urine level is higher. • Level of hCG return to normal 2 weeks after delivery and 3-5 weeks after abortion.

  17. Prostate Specific Antigen (PSA) • 50-90% bound to protein & remainder is free. • PSA has clinical application as a tumour marker for screening, Dx and management of prostate cancer. • PSA lacks specificity for cancer because it is affected in benign prostatic condition. • Screening for prostate cancer has not ↓ morbidity or mortality. • PSA ↑ in BPH, urethral instrumentation, prostatitis, TURP, Acute urinary retention, prostate ischaemia, cystoscopy.

  18. Thyroid Profile • TSH is the best test to confirm or exclude thyroid disease. • Normal level 0.5-5mU/L. • TSH <0.1mU/L suggest hyperthyroidism • Level from 0.1 mU/L to the 0.5mU/L are indeterminate & may represent autonomous thyroid hormone production or excess thyroid hormone administration. • Level from 6-15 mU/L are often considered as subacute hypothyroidism and are not usually associated with symptoms. • Symptomatic hypothyroidism occurs with TSH > 15 mU/L. • Conditions that decrease TSH include excess thyroid hormone either endogenous or exogenous, sever illness & acute glucocorticoid therapy. • Conditions that cause ↑ TSH include hypothyroidism, severe acute illness, iodine deficiency, adrenal insufficiency and depression.

  19. Alkaline Phosphatase • It is found in liver, bone, intestine and placenta. • Elevation is seen in normal childhood and adolescence as well as pregnancy . • Normal range in adults 25-100 U/L. • Mild elevation (1-2 times normal) can occur in parenchymal liver disease e.g hepatitis. • Marked elevation occurs in infiltrative liver disease or biliary obst. GGT ↑ in obstructive liver disease. • When elevated GGT is associated with normal alkaline phosphatase inquire about drugs & medication use (ethanol, warfarin or other anticonvulsant use all of which ↑ GGT).

  20. Liver Enzymes (Aminotransfrases) • Liver enzymes are not specific for liver abnormalities. • Insult to liver including inflammation, necrosis or biliary obstruction can result in release of hepatic enzymes. • Transaminases are used as markers for hepatocellular damage, chronic hepatic dysfunction can lead to decrease synthesis of protein and cause prolong PT. • PT is better marker for acute liver disease than albumin. • AST (SGOT), ALT (SGPT) modest elevation can be seen in metastatic or infltrative liver disease.

  21. Marked elevation suggests necrosis as seen in viral or drug induced hepatitis or shock state. • With liver disease AST and ALT are elevated to similar degree. • In alcoholic hepatitis AST are approx twice ALT. • Also AST ↑ and of ALT in Wilson disease, fatty liver of pregnancy and in shock liver. • AST found in skeletal muscle, kidney, heart where as ALT found in liver, so elevation of ALT are more specific to liver disease compare to AST.

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