Chemical Pathology JC134: Point-of-care Testing (POCT)

Question 1

Point-of-care testing

Answer 1

POCT:
- Any test performed **at the time at which the test results enable a decision to be made + action taken that leads to improved health outcome
- Characteristics:
—> **NOT done by lab but doctors / nurses

General considerations:

1. SOP
2. Training + certificate
3. QA
4. Internal QC
5. External quality assessment
6. Documentation + reporting
7. Maintenance
8. Safety

Question 2

Advantages of POCT

Answer 2

ALL need to based on **accuracy + **reliability of POCT results

1. ***Reduced TAT (Turnaround time)
2. Improved patient management
3. Reduced admin work + test request
4. ***Minimise delay of sample collection
5. ***Minimal sample requirement
6. Reduce time delay of transport
7. Reduce time delay of results (register / log in sample)
8. Reduce time delay from entry of sample into a complex facility
9. Overcome limitation of traditional testing (which is centralised)
10. Transportation: disconnection between time, individual, specimen, laboratory, physician
11. Portable
12. Dry, stable, unit-dose device
13. Robust (ease of storage + usage)
14. ***Simple to use
15. Concordant result with central lab
16. Safely operated (no mechanical parts)
17. Built-in / Integrated calibration / QC (i.e. all standardised)
18. Ambient storage temp for storage of reagents
19. ***Low instrument cost
20. Record keeping (through internet / other means)

Question 3

Examples of POCT

Answer 3

1. Blood gas analysers
2. Blood gas / electrolyte / metabolite handheld meters
3. Haemostasis analysers
4. Cardiac marker analysers
5. Urine test strips (Urine dipstix): visual + readers
6. Glycated Hb analysers
7. Whole blood glucose + lactate meters
8. Pregnancy tests
9. Faecal occult blood
10. HIV testing

Question 4

***Interpretation of urine dipstick analysis

Answer 4

Pathologic + Non-pathologic causes of abnormalities of dipstick results

1. Low SG (specific gravity)
   Pathologic: DI, Renal tubular dysfunction
   Non-pathologic: Polydipsia
2. High SG
   Pathologic: Volume depletion
   Non-pathologic: Inadequate volume intake
3. Low pH
   Pathologic: Acidosis
   Non-pathologic: ***High protein diet
4. High pH
   Pathologic: ***Renal tubular acidosis (inappropriate renal response), UTI
   Non-pathologic: Low protein diet, Recent meal
5. Blood
   Pathologic: Glomerular disorders, Tubular disorders, UTI, Stones, Hypercalciuria, UT trauma, Tumour
   Non-pathologic: Menses, Traumatic catheterisation, Exercise
6. Protein
   Pathologic: Glomerular disorders, Tubular disorders, UTI
   Non-pathologic: **Orthostatic proteinuria, **Fever, ***Exercise
7. Glucose
   Pathologic: DM, Fanconi syndrome
   Non-pathologic: Renal glycosuria
8. Ketones
   Pathologic: DM
   Non-pathologic: ***Restricted CHO intake
9. Bilirubin
   Pathologic: Hepatitis, Biliary obstruction
   Non-pathologic: None
10. Urobilinogen
    Pathologic: Hepatitis, IV haemolysis
    Non-pathologic: Low: ***Systemic antibiotic therapy
11. Nitrite
    Pathologic: ***UTI
    Non-pathologic: None
12. LE (Leukocyte esterase)
    Pathologic: **UTI, **GN, **Pelvic inflammation
    Non-pathologic: **Fever

Question 5

Urine ketone

Answer 5

Principle:
Nitroprusside reaction-based methods
- Acetoacetate (measured)&raquo_space; Acetone (not really measured)
- β-Hydroxybutyrate (reduced formed of Acetoacetate) NOT measured

Question 6

***False positive + False negative results for urine dipstick analysis

Answer 6

記: Ascorbic acid (Reducing agent)會令好多都False negative

1. SG
   False positive: ***Contamination during collection / storage
   False negative: None
2. pH
   False positive: High pH from **urease producing organisms (e.g. Proteus mirabilis), **Prolonged standing of urine
   False negative: Low pH from ***mixing of reagents from adjacent test pads
3. Blood (記: oxidised左就有血)
   False positive: Oxidising contaminants (e.g. hypochlorite), Microbial peroxidase associated with UTI
   False negative: High ascorbic acid (reducing agents), Large **nitrites, **High SG
4. Protein
   False positive: **Fever, Exercise, Alkaline urine, **Concentrated urine, Presence of cells / bacteria in urine
   False negative: **Dilute urine, **Low molecular weight proteins
5. ***Glucose (記: oxidised左就有糖)
   False positive: Strong oxidising agents in urine container
   False negative: Ascorbic acid (Reducing agents), High SG, Exposure to humid environment
6. **Ketones
   False positive: Captopril, Methyldopa
   False negative: **Prolonged standing of urine (∵ Acetoacetate is unstable —> metabolised into Acetone), Moisture on test pad
7. Bilirubin
   False positive: **Rifampicin, Chlorpromazine
   False negative: **Ascorbic acid, Prolonged standing in light
8. **Urobilinogen
   False positive: Alkaline urine, **Sulfonamides
   False negative: ***Broad spectrum antibiotics, Discoloured urine, Prolonged standing in light
9. Nitrite
   False positive: Urine contamination, Medications that turn urine colour to red, Gross haematuria
   False negative: Inadequate dietary nitrate intake (vegetables), ***Non-nitrate-reducing bacteria, Insufficient incubation time for conversion of nitrate to nitrite, Ascorbic acid, High urobilinogen

(Nitrate-reducing bacteria (CPRS75):
Enterobacteriaceae, Staphylococcus, Proteus, Pseudomonas)

10. LE (Leukocyte esterase) (記: oxidised左就有LE)
    False positive: Contamination with vaginal fluid, **Oxidising agents, Trichomonas
    False negative: **Ascorbic acid, High protein, High glucose, ***High SG, Cephalosporins, Tetracycline, Nitrofurantoin

Question 7

Urine “pregnancy” test

Answer 7

• Lower limit of detection of urine hCG: ***25 IU/L (varies in different test kits)
• Blood hCG: Non-pregnant <5 IU/L

Gestational age:

• 0-1 week: 5-50
• 1-2 weeks: 50-500
• exponential increase in 1st trimester

Interpretation:
- Must have BOTH **Testing band + **Control band (quality control)

Limitations:

• Not sensitive in detecting very early pregnancy
• Hook effect (***太多HCG令到造唔到sandwich formation) causing False -ve results
• Hook-like effect (***variants of HCG not recognised by one of sandwich Ab —> 都係令到造唔到sandwich formation)

Question 8

HCG variants and POCT devices

Answer 8

Common HCG variants:
Urine + Serum:
- HCG (intact HCG)
- HCG-h (hyperglycosylated HCG)
- HCGn (nicked HCG)
- HCGβ (free β subunit)

Urine only:
- HCGβcf (core fragment of HCGβ)

—> Different POCT device may recognise different variants (different pregnant lady produce different variants of HCG)
—> Negative does ***NOT mean patient is not pregnant

Question 9

Causes of Elevated hCG / Positive pregnancy tests

Answer 9

1. Normal pregnancy
2. Abnormal pregnancy (ectopic pregnancy, miscarriage)
3. Gestational trophoblastic disease (e.g. Choriocarcinoma)
4. Germ cell tumours (in both male + female)
5. Ectopic hCG producing tumours (e.g. Ca lung)
6. Pituitary hCG (normal, increased in post-menopausal women)
7. Previous injection of hCG (by athletes / bodybuilders)
8. Immunoassay interferences
   (9. Cross-reactivity with LH / its subunit (unlikely a problem with the currently used 2-site immunometric assay))

False positive:

10. Biochemical pregnancy (early miscarriage)
11. Missed reaction time / Incorrect reading time (test sit for too long —> may cause false positive results)
12. Drugs (e.g. HCG shot for infertility treatments)
13. Evaporation lines (some pregnancy tests leave a faint evaporation line as the urine crosses the test —> misinterpret as positive)
14. Mislabelled specimens

Question 10

Testing for Drug of abuse

Answer 10

Examples: Cocaine, Cannabis

Positive POCT —> Must also send urine sample to laboratory for ***confirmation (∵ POCT many false positive / negative results)

Question 11

Urinalysis

Answer 11

Evaluate sample of urine
- used to detect + assess wide range of disorders (e.g. UTI, kidney disease, DM)

Examination of:

1. Appearance
2. Concentration
3. Content

Abnormal results of urinalysis often require more testing + evaluation to uncover source of problem

Question 12

Blood glucose meter (Glucometer)

Answer 12

• To determine approximate blood glucose by small drop of blood, pricking from skin by a lancet
  —> blood placed on test strip
  —> for home monitoring of blood glucose in patients with DM / hypoglycaemia
  —> need Calibration

MOA:
Enzymatic reaction of test strip (Redox reaction)
- Glucose Oxidase
- Glucose Dehydrogenase
—> Pyrroloquinoline quinone (PQQ)
—> Flavin adenine dinucleotide (FAD)
—> Nicotine adenine dinucleotide (NAD)

• **GDH-PQQ:
• non-selective for glucose —> also detects other non-glucose sugars: maltose, xylose, galactose, icodextrin (in PD fluid), immunoglobulins

Question 13

Hand-held Glucometer / Haemoglucostix

Answer 13

• Poor analytical accuracy at both low + high glucose levels
• Usual dynamic range (reliable range): 2.5-28 mmol/L
  —> **dangerously low glucose level (<2) can be missed (still give 2.5 glucose reading)
  —> always send a concomitant **fluoride-oxalate blood sample to main lab for verification
• Take concomitant blood samples for **Insulin, **C-peptide, **Cortisol, **GH etc. if hypoglycaemia is clinically evident
  —> but don’t delay any clinical interventions to await the results
  —> treat hypoglycaemia straightaway after blood sampling

Question 14

Potential sources of error of glucose meter

Answer 14

Patient sources of error:

1. Medications (e.g. maltose, xylose, galactose)
2. Haematocrit
3. Disease contraindications (e.g. poor circulation, DKA, trauma)

Environmental sources of error:
1. Extreme temp

Operator sources of error:
1. Expired reagents
2. Incorrect calibration
3. Incorrect sample volume
4. Inadvertent change of units
5. Incorrect disinfection
Etc.

Question 15

POCT blood gas analyser

Answer 15

Based on electrolytes

• Arterial blood specimen
• Avoid air bubbles
• Label specimen with GUM label
• Refrigerate specimen if analysis not done immediately
• Blood gas analysis must be performed within 90 mins