by Dr. Adele Visser
Introduction
The use of the urine dipstick within a point of care setting, can provide a clinician with a lot of valuable information in the context the patient’s health.
This coupled with laboratory analysis of urine (urinalysis), may facilitate early diagnosis and correct treatment or patients with a host of different conditions.
Most commonly used within the setting of urinary tract infections (UTI’s), it can also be applied as screening for metabolic, hepatic, renal and haematological disease. Notably, UTI’s account for as much as 1 – 3% of consultations by adult patients in general practice1 and for up to 13,7% of antibiotic prescriptions2, highlighting its importance as a community health issue.
In many instances, the first line evaluation of the patient would be through the POCT urine dipstick method. It holds the advantage of 99% specificity but 45% sensitivity for UTI if positive for urine leucocyte esterase and nitrites but cannot provide information as to the causative pathogen3.
Urine dipstick tests are the most commonly used clinical test in the diagnosis of patients with urinary tract infection, as they are cheap and simple to operate. Because quantitative urine culture takes a long time, a urine dipstick test can be performed first and then confirmed by microscopy and urine culture. The combination of these 2 test methods can effectively improve the detection rate, reduce the false positive rate and most importantly allow for targeted treatment and reduce antibiotic misuse4.
This article serves to highlight the essential points around POCT and laboratory urinalysis and how it should be applied in clinical practice. It also attempts to show the shortcomings of these tests and therefore areas where discrepancies in results may occur.
Tests | Results | Testing method | Pre- and Analytical considerations |
Microbiology | |||
Specimen Site | Urine sample, light yellow, turbid | Macroscopic, visual evaluation | |
Urine Chemistry | |||
U-pH | 5 | Tetrabromophenol blue or tetrachloropheno | Delay in transport. Reagent overflow from protein patch. |
Urine MCS | |||
U-Leucocytes | Trace | Labile neutrophils release leucocyte esterase. Enzymatic measure with colour correlating to concentration | False positives (FP) with vaginal fluid contamination. |
U-Nitrate | Negative | Certain classes of bacteria reduce nitrates to nitrites. Requires 104 to 105/mL organisms and time to convert = therefore useful only in first morning sample. | Discrepant POCT and Laboratory results possible with delay or certain bacteria. FN – low nitrate diet, urobilinogen. |
U-Urobilinogen | Normal | Uses Ehrlich aldehyde or Redazo dye methods. | Labile and requires a fresh sample. Discrepant POCT and Laboratory results possible. |
U-Protein | Negative | Tetrabromphenol blue typically used. More sensitive for albumin than other proteins | False positives with alkaline or quartarnary amine contamination (disinfectants). |
U-Blood / Hb | Trace | H2O2 chromogen method used. Will pick up 0,3mg Hb/mL (approx. 10 red blood cells). | Tests positive with haemoglobin and myoglobin. |
U-Specific Gravity | 1.020 | Indirect polyelectrolyte indicator. | Not affected by glucose, protein or contrast media. |
U-Ketones | Negative | Nitroprusside method. Most sensitive to aceto-acetic acid then acetone. | Does not react with 3-hydroxybutyrate. |
U-Bilirubin | Negative | Diazo method. Bilirubin rapidly hydrolysed therefore fresh sample is essential. | Discrepant POCT and Laboratory testing may occur due to delay. FP – Rifampicin. |
U-Glucose | Negative | H2O2 chromogen method. Sensitive to only glucose. | Not adequate in newborn screening as all reducing sugars need to be detected. |
Urine Microscopy | |||
U-Leucocytes | 8 / hpf | Neubauer counting chamber used. Counting 9 fields x1000 to quantify it per mL. | FN – delay may lead to cells lysis. |
Red bloods cells | 3 / hpf | Neubauer counting chamber used. Counting 9 fields x1000 to quantify it per mL. | FN – delay may lead to cells lysis. |
Epithelial Cells | ++ | Neubauer counting chamber used. Semiquantitative reporting. |
Tests | Results | Testing method | Pre- and Analytical considerations |
Casts | |||
Hyaline | Present | From protein in renal tubules. | |
Granular | Absent | Degenerated cellular material. | |
Cellular | Absent | Degenerated and intact cellular material. | |
White Blood Cells | Absent | Indicate infection. | Pyelonephritis or glomerular disease. |
Red Blood Cells | Absent | Due to severe glomerular injury. | Acute glomerulonephritis Lupus. Bacterial endocarditis and septicaemia. |
Urine Crystals | |||
Amorphous Urates | Present | No pathological significance. | |
Amorphous Phosphates | Absent | No pathological significance. | |
Calcium Oxalate | Absent | Ethylene glycol poisoning and high oxalate intake in diet. | |
Uric Acid | Absent | High serum uric acid or uric acid renal stones present. | |
Triple Phosphate | Absent | Associated with certain diets. | |
Trichomonas vaginalis | Absent | Infection. | |
Schistosoma haematobium | Absent | Infestation. | |
Urine Culture | |||
Culture Comment | See identification below | Full identification with sensitivity profile reported. | |
Antimicrobial substances | Present | Suppression of growth of Bacillus subtilis indicate antibiotic presence. |
Conclusion
The combination of POCT and formal urinalysis serves a supplementary function in the evaluation of a patient’s clinical condition.
Comprehending the shortcomings of each platform enables more prudent interpretation of results and ultimately the effective treatment of the patient.
References
- Schmiermann G, Kniehl E, Gebhardt,K et al. 2010. The Diagnosis of Urinary Tract Infections – A Systematic Review. Deutsches Arzteblatt International. 107(21): 361-7.
- Health Protection Agency. English National Point Prevalence Survey on Healthcare-associated Infections and Antimicrobial Use, 2011: Preliminary data. Published 2012. Accessed Jan 20, 2016.
- St John A, Boyd JC, Lowes AJ, Price CP. 2006. The use of urinary dipstick tests to exclude urinary tract infection: a systematic review of the literature. Am J Clin Pathol. 126(3):428-36.
- Xie R, Li X, Li G, Fu R. Diagnostic value of different urine tests for urinary tract infection: a systematic review and meta-analysis. Transl Androl Urol. 2022 Mar;11(3):325-335. doi: 10.21037/tau-22-65. PMID: 35402195; PMCID: PMC8984977.