Lab Workup to diagnose diseases, Types of samples, Factors Effecting the Tests
Lab Workup to diagnose diseases
- There are various tests to diagnose the diseases. We can discuss one by one of these tests to diagnose the diseases.
Various types of tests:
- There are various tests done to diagnose and monitor the disease. These are as follows:
- Screening test:
- These are the simplest way to suspect diseases like sugar in the urine indicating diabetes.
- Raised levels of PSA may need to rule out the possibility of prostatic cancers.
- Fecal occult blood may need to rule out colon cancer.
- Diagnostic tests:
- These are specific tests to confirm the disease, like colon biopsy to confirm colon cancer.
- ECG to confirm heart disease.
- Hormones like T3, T4, and TSH diagnose thyroid diseases.
- Monitoring or follow-up tests:
- These tests are done to know the progress of the disease.
- CA 125 may increase the uncontrolled spreading of ovarian cancers.
- ESR may become normal in patients with tuberculosis.
- Serial estimation of liver enzymes gives the idea of improving or deteriorating liver function.
- Surgical procedures:
- These are like a biopsy, or FNA ( fine needle aspiration cytology ) may give the diagnosis of cancers or benign lesions.
- A liver biopsy may give us the grades of chronic hepatitis or malignancy.
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FNA of the breast is a very useful tool for diagnosis with an accuracy of above 95 % benign or malignant breast diseases.
- Radiological procedures:
- Nowadays, radiological procedures are advanced to diagnose various diseases of the bone, like benign osteochondroma or malignant osteogenic sarcoma.
- Barium meal or Barium enema can diagnose GIT diseases.
- Ultrasonography:
- This specialty has progressed a lot. It is a non-invasive procedure and helps diagnose any mass in the liver, ovary, or any other part of the body.
- This can also be used for the follow-up of tumors.
- This can be used to guide the FNA procedures.
- CT scan:
- This is computerized tomography, and it gives more detailed information than a simple X-RAY.
- This can also give a better estimate of the size of tumors.
- This is a very useful tool for diagnosing brain pathology like CNS tumors.
- MRI:
- This is magnetic resonance imaging and is more accurate than a CT scan.
- It can detect a tumor size of even 5 mm in diameter.
Examination of various body fluids:
- Body fluids like CSF, ascitic fluid, or Pleural fluids can also help diagnose various diseases.
- Like ascitic fluid, cytology may tell you the presence of malignant cells.
- CSF examination differentiates various types of meningitis.
Types of samples collected from the patient:
- There are the following body fluids samples used in the routine.
- Venous blood:
- Venous blood is easy to collect, telling the body’s physiologic condition.
- Most tests like LFT, Kidney function tests, CBC, and other biochemical tests are done on venous blood.
- This sample can be collected using a sterile needle from any prominent veins like arms and hands.
- Risk: The major risk in this procedure is a local hematoma. and infection is almost nonexistent.
- Precaution: Don’t take the blood when the patients have an intravenous infusion.
- A fasting blood sample means a fast of 8 to 12 hours when no food or liquid is taken.
- The postprandial blood sample is taken after a meal.
- 2 hours of postprandial blood is taken in the patient with diabetes mellitus.
- Arterial blood:
- Arterial blood is needed for the estimation of blood gases.
- The blood sample is usually taken from the femoral artery in the groin.
- The radial artery at the wrist.
- The brachial artery at the elbow.
- The neonates can get blood from the umbilical artery.
- Taking blood from the artery needs an experienced person like a doctor, trained nurse, or technician.
- Capillary blood:
- Capillary blood is taken in newborns or infants.
- Usually, the prick is done on the fingers or heel.
- Serum:
- The whole blood is allowed to clot, where all the clotting factors are removed by clotting.
- The best way is to keep the blood at 37 °C for 20 to 30 minutes, then clear serum will come up, leaving clotted blood at the bottom; or you can centrifuge the blood to get the clear serum.
- Procedure to get good serum:
- Take blood in the test tube, leave for 20 to 30 minutes, and then centrifuge for 2 to 5 minutes, and you will get the clear serum.
- Not good serum:
- If the serum is reddish, discard it because it shows hemolysis.
- Hemolyzed serum interferes with the various tests.
- The serum is usually used for biochemical tests, hormones, and tumor markers.
- The whole blood is allowed to clot, where all the clotting factors are removed by clotting.
- Plasma:
- Plasma is obtained from the blood, which contains an anticoagulant.
- Mix anticoagulant into the blood and centrifuge it. We will get a clear plasma at the top and bottom with RBC, WBC, and some clotting factors and proteins.
- This can be used for various tests but is not a good substitute for serum.
- Pleural fluid:
- This fluid is present in the pleural space.
- The needle takes this fluid out, and the procedure is called Thoracentesis.
- This fluid is analyzed to diagnose malignancy and may be used for culture and biochemical tests.
- This fluid is present in the pleural space.
- Ascitic fluids:
- This fluid is collected from the abdominal cavity in case of ascites.
- This ascitic fluid may be analyzed to diagnose benign or malignancy. Also, some biochemical tests are done.
- Synovial fluids:
- This is the joint fluid aspirated aseptically, and its analysis diagnoses its formation.
- Cerebrospinal fluid:
- This fluid is present in the spinal canal and brain.
- It is aspirated from the spinal canal aseptically.
- A physician or anesthetist usually does this procedure.
- This fluid is used to diagnose various types of meningitis and can be cultured to find the causative organism.
- Urine:
- Random samples may be taken for urine analysis.
- Or you can collect 24 hours samples for urinary protein, urea, creatinine, or other biochemicals..
- Random samples may be taken for urine analysis.
Factors affecting the lab tests:
- Hemolysis:
- Whenever there is traumatic venesection that leads to hemolysis due to rupture of the RBC.
- These RBCs release various enzymes and chemicals which interfere with the results. e.g., Potassium is quite high in haemolysed serum.
- Anticoagulants:
- If added in, an increased amount will affect the results.
- Temperature:
- Some tests are sensitive to a temperature where you may need frozen serum or plasma to preserve those chemicals. e.g., the ACTH level needs frozen plasma.
- While some tests need a serum to be at room temperature.
- Some tests are sensitive to a temperature where you may need frozen serum or plasma to preserve those chemicals. e.g., the ACTH level needs frozen plasma.
- Fasting samples:
- Some of the tests need fasting samples, like fasting glucose levels.
- We need a fasting sample for the Lipid profile because you will get higher values on the random sample.
- Some of the tests need fasting samples, like fasting glucose levels.
- Drugs interference:
- Some drugs like aspirin, vit C, cold tablets, pain reliever, and antacids alter the test result.
- Sleeping habits and diet:
- It may alter the level of the hormone.
- Diurnal variation:
- Keep in mind the diurnal variation of some of the tests.
- Exercise:
- It may affect the test results.
- Food intake like:
- Food containing caffeine which may affect the tests.
- Smoking containing nicotine will affect the tests.
- Alcohol intake will affect the tests.
Blood sample-taking method:
- If you apply a tourniquet and take blood for calcium, the values will not be correct.
- If you squeeze the finger to get an adequate amount of blood, the values will be less and not accurate because of the dilution effect.
- While interpreting the values of a test, keep in mind age, sex, height, and weight.
Precautions before performing the lab tests:
- This is best to perform all the tests within the first 45 minutes to one hour after collection.
- Separate the plasma as soon as possible, don’t delay more than two hours.
- Premature separation of the serum will have microclots. It may block the pipettes.
- Coagulation in the plastic containers is delayed more than 20 to 30 minutes.
- If delayed in performing the test, properly store the sample at the appropriate temperature.
- If the serum’s separation is delayed, keep the blood at room temperature to avoid hemolysis.
- If the test is not performed immediately, keep the serum at 4 °C.
- Centrifuge the tubes with the cap on them; otherwise, there will be evaporation.
- The serum is needed for electrophoresis.
- For gases and ammonia, whole blood is needed.
- The serum is the best option for clinical chemistry, but plasma may be used.
- For plasma preparation, centrifuge the blood within one hour of sample collection for 10 minutes at 850 to 1000 X gravity.
- Caped the bottle to avoid evaporation.
Abdominal tap for ascitic fluid:
- In this procedure is advised to tap peritoneal fluids.
- Usually, a 20 to 18-gauge needle is used.
- Usually, use the lateral side of the abdominal wall, lateral to the umbilicus; this site is sterilized before the procedure.
- One of the useful values in the peritoneal fluid is protein level which is low in transudate < 3 g/ dl and high in exudate > 3 g /dl, which may be due to inflammation or malignancy.
- Unclotted blood indicates hemorrhage in the peritoneal cavity.
- The presence Of inflammatory cells suggests Pancreatitis, appendicitis, rupture of organ, or malignancy.
- Normally there are few mesothelial cells.