INTERPRETING LABORATORY RESULTS

With continuous medical education & research we have identified newer risk factors, etiologies & management strategies which has led to the current concept of “Evidence Based Medicine” & “Laboratory Medicine.

Simultaneously, there are newer techniques & methods which are made available for investigating these new factors, there by aiding the clinician for disease diagnosis as well as its management.

Hence, “Disease Diagnosis” today is “Investigation Based”.

What do Unexpected Results Mean ? Thus, when confronted with an unexpected or abnormal test result one should

take into consideration the various test variables which can contribute & there by explain the result variation apart from laboratory error & change in patient’s clinical status.

Laboratory Testing- Complex Process

Interpreting laboratory tests –For evaluating these test results, one has to develop a systematic approach for their correct interpretation & this would include - A good clinico-pathological correlation - Recognizing & understanding the conditions required for testing a particular biologic parameter.

PRE-ANALYTICAL VARIATIONS

These can be broadly divided into three categories

1. Physiological factors

2. Specimen collection & handling

3. Endogenous variables –like drug interferences, circulating antibodies

PRE-ANALYTICAL VARIATIONS

1. PHYSIOLOGIC / BIOLOGIC VARIATIONS

• These can be subdivided into

- Diurnal Intraindividual physiological

variations (Serial tests)

- Age related variable reference ranges

(Neonate\Child\Adult\Geriatric)

ANALYTES THAT DISPLAY CONSIDERABLE BIOLOGIC VARIATION -

TEST AGE RELATED BIOLOGIC VARIATION DIURNAL PHYSIOLOGIC VARIATION

Potassium - 1.0 – 1.2

Creatinine - 0.04 – 0.20

Glucose Increases by 20-30% in > 45 yrs 5.8 – 26.0

Magnessium - 0.04 – 0.2

Albumin Decreases upto 2 %> 45 yrs 0.1 – 0.4

AST Increases upto 20% for men& upto 30% for women > 45 yr

5 -20

ALT 5 - 28

Calcium - 0.2 – 0.7

Cholesterol Increases upto 30-40mg%> 45 yr 20 - 36

Triglyceride Increases by 30% > 45 yrs 30 - 100

CK - 40 - 60

LDH - 50 - 90

Amylase - 20 - 45

Hemoglobin Decreases by 1-2% > 45 yrs 0.4 – 0.9

Hct Decreases by 1-2 % > 45 yrs 2 - 3 %

WBC Decreases by 5 % > 45 yrs 500 - 1900

Platelets - 9,000 – 12,000

PT-INR - 0.19 – 0.84

PRE-ANALYTICAL VARIATIONS

2. Specimen collection & handling –

- Test pre-requisites - Phlebotomy & blood collection - Type of sample - Blood anticoagulation - Quality of sample - Transport of sample

Test pre-requisites

• 24 hr Urine collection in prelabelled container with necessary preservatives.

• Urinary 17-keto steroid, 17-OH corticosteroid, 5-HIAA, VMA, porphobilinogen require HCl 10-15 ml

• Urinary Proteins, uric acid, creatinine require boric acid 15 gms

Test pre-requisites

Test pre-requisites

1. Sample collection site - preferably other than the IV infusion site & in appropriate pre-labeled anticoagulant vial\tube. Use of serum separator tubes help in easy, fast separation of serum & reduce the serum-clot contact time

2. Order of draw affects the quality of the sample and can lead to erroneous test results due to contamination with the additive from the previous collection tube (CLSI H3-A5 standard)

Phlebotomy & blood collection

TEST % INCREASE

Total protein 4.9

Cholesterol 6.7

Creatine Kinase 5.8

AST 9.3

CHANGES IN TEST RESULT WHEN PHLEBOTOMY TOURNIQUET IS PROLONGED FROM 1 MIN TO 3 MINS

Phlebotomy & blood collection

CHANGES IN TEST RESULT WHEN PHLEBOTOMY PERFORMED USING TOURNIQUET- FOR TEST PARAMETERS CALCIUM & CPK

TEST ARTERIAL CENTRAL VENOUS

PERIPHERAL VENOUS

ALT 62 61 81

AST 20 20 32

Albumin 3.6 3.7 3.9

Creatinine 1.4 1.3 1.1

Potassium 4 3.9 3.6

Creatine Kinase 80 78 94

Platelet count 1.0 0.90 1.24

Capillary value > venous Capillary value < venous

Glucose 1.4% Bilirubin 5.0 %

Potassium 0.9% Calcium 4.6 %

Total protein 3.3 %

Type of sample

Blood anticoagulation

Sample Quality –

Falsely high Falsely low

Hemolysis LDH, AST, ALT,CK,Potassium

Bilirubin ,Albumin,Alk PO4,γ- GT

Lipaemic Hemoglobin, total proteins Creatinine, Urea,AST, ALT, CK

Icteric Hemoglobin, total proteins Creatinine, Albumin, Amylase

Transport of sample

Factors to be considered during sample transport are1. Transit time - Eg. Serum PTH, Plasma lactate, plasma ammonia2. Temperature- Eg. CD4 count, Coagulation & Hormonal assays3. Packaging – to avoid any leakage & maintain Cold chain

PRE-ANALYTICAL VARIATIONS

3. Endogenous variablesDrugs can cause analytical interferences – for common analytes like Hb, glucose, total proteins, s. creatinine & serum electrolytes.

They cause interferences by- changing the serum or plasma matrix (i.e. reduction in proteins, increase in

co volume of solutes(osmolality), redistribution of electrolytes(pH))

(eg.IV fluids)- sometimes in vitro hemolysis (eg. Salicylates) - enzyme inhibition(eg.cephalosporins, aminoglycosides & creatinine,

vitamin C & G6PD).

Circulating antibodies –

Especially autoantibodies(eg. SLE,RA), paraproteins(eg. CLL/SLL, Myeloma) can cause spurious results for uric acid, creatinine, WBC & Platelet counts

ANALYTICAL VARIATIONS

• These contribute minimally in the test variation because– Manual methods are replaced by Automation– Strict Quality Control (Internal & External)

Factors contributing in this variation are- Quality of water used for reconstitution of

reagents/calibrators/controls or probe rinsing- Stability of reagents/calibrators/controls & their

appropriate storage - Test Reagent lot variation- Test Calibration variation - Random Instrumental malfunction

Post-Analytical Variables – Solutions

• Data entry errors• Oral miscommunication of results• Electronic medical errors• Failure to communicate critical values

POST ANALYTICAL VARIATION

Total Test Variation

Biologic parameter

Total Test Variation %

Sodium 1.9

Potassium 7.1

Chloride 3.8

Calcium 3.2

Creatinine 14.5

Uric acid 11.5

Total protein 4.8

Albumin 5.5

AST 25

ALT 56

ALK PO4 20

LDH 16

Cholesterol 14.8

• A study on 21 subjects with serial testing on routine biochemistry analytes with samples drawn three times in a day & analysed for preanalytical, analytical & post-analytical variations.

Ref –Textbook of clinical chemistry ,Teitz, pg 58-60

Case studies• Case 1: A Child with an Isolated Elevation

in Alkaline Phosphatase (AP)

A 4-year-old child has a serum AP of 1150 U/L,serum phosphate of 5.0 mg/dL and serum gamma-glutamyltransferase (GGT) of 30 U/L. The physical examination of the child is unremarkable except for a painful bruise on the right leg.

On investigation - The serum was visibly hemolyzed owing to difficulty in obtaining the specimen.

• Case 2: An Asymptomatic Woman with Hypercalcemia

An asymptomatic 65-year-old woman on a routine physical examination is noted to have the following abnormalities in her biochemical profile:

Serum calcium = 11.3 mg/dL

Serum phosphorous = 2.9 mg/dL

Serum sodium = 140 mEq/L

Serum potassium = 4.0 mEq/L

Serum chloride = 112 mEq/L

Serum bicarbonate = 16 mEq/L

On investigation – the blood collection time exceeded one minute & touniquette used

Case studies• Common problem is with an isolated elevation in a laboratory test – Whether it represents a true- or a false-positive result?

The latter being more likely owing to the low prevalence of disease in ambulatory patients.

Although most clinicians repeat the test when there is doubt about its validity.

An alternative approaches are –

1. Recall that the reference intervals for the majority of laboratory tests are based on 2 SD from the mean of the test. Recalculate what the reference interval of the test would be using 3 rather than 2 standard deviations (SD) from the mean of the test.

eg. The mean of the serum ALT is 14 U/L (normal, 8-20 U/ L), hence 2 SD is 6 U/L and 1 SD is 3 U/L. Since 3 SD encompasses 99% of the normal population, the upper limit of the ALT reference interval for 3 SD is 23 U/L (14 + 9).

2. Correlating with other associated tests

3. Using updated medical guidelines for result interpretation.

CONCLUSION-

- When confronted with an unexpected differing test result , clinicians should take into consideration these intraindividual biologic, preanalytical & analytical variations before interpreting them.

- Awareness & recognition by the clinician of these variations in an individual, which contribute in the total test variation would go a long way in a better health care management of the patients.

REFERENCES –

* 1. Clinical Diagnosis & Management by Laboratory Methods ; Todd & Sanford, Henry XX Edition.

* 2. Teitz Textbook of Clinical Chemistry ; III Edition.

*3. Interpretation of Diagnostic Tests ; Jaques Wallach, VI Edition.

* 4. New in vitro analytical approches for clinical chemistry measurements in critical care ;Clin. Chem. 1990 Aug; 36(8 pt 2) : 1567 - 72

* 5. What do unexpected Results mean ? ;Postgrad. Med. 2000 June; 107(7); 145-162.