MSU (Mid-stream Specimen of Urine) at Dublin health Screening
What is the purpose of an MSU test?
- To confirm the diagnosis of a urine infection. The usual symptoms of a urine infection are pain when you pass urine, and passing urine frequently. However, symptoms are not always typical, particularly in children and the elderly, and a urine test is needed.
- To decide the best antibiotic to use. Some bacteria (germs) are resistant to some antibiotics. If the test shows that bacteria are in the urine then the bacteria are tested against various antibiotics. This finds which antibiotics will kill the bacteria in the urine.
How do I do a mid-stream specimen of urine?
The aim is to get a specimen (sample) of urine from the middle of your bladder. Urine is normally sterile (no bacteria present). If bacteria are found in the sample, it means that the urine is infected. A ‘mid-stream’ sample is best as the first bit of urine that you pass may be contaminated with bacteria from the skin.
Women – hold open your labia (entrance to the vagina). Men – pull back your foreskin. Pass some urine into the toilet. Then, without stopping the flow of urine, catch some urine in a sterile bottle. (The bottle is usually provided by a doctor or nurse.) Once you have enough urine in the bottle, finish off passing the rest of your urine into the toilet.
Do not open the sterile bottle until you are ready to take the sample. You do not need to fill the bottle to the top, a small amount will do. (Some specimen bottles contain a preservative. If this is the case, a mark on the bottle will indicate the ideal amount of urine. However, if that is difficult, any amount is better than none.)
- Positive test indicates either haematuria, haemoglobinuria or myoglobinuria.
- Dipstick test for presence of haemoglobin with degree of colour change directly related to amount present.
- Can appear as both coloured dots and change in the colour field.
- False positive readings are most often due to contamination with menstrual blood. Incidence of false positives can be increased by dehydration which concentrates the number of RBCs produced and exercise.
- Haematuria is defined as >3 RBC/high power field (hpf) of centrifuged sediment under microscope.
- Dipsticks are 90% sensitive but somewhat less specific.
- Prognostic significance of positive test is very controversial, rates ranging from 0.5 – 6% of patients with positive test have been found to have underlying significant pathology.
- Healthy adults normally excrete 80-150mg protein in urine daily.
- Detectible proteinuria may be first sign of renovascular, glomerular or tubulo interstitial renal disease.
- Alternatively may be caused by overflow of abnormal proteins in diseases such as multiple myeloma.
- The dipstick detects presence of protein by increasingly darker shades of green. Minimum detectible protein concentration is 20-30mg/dl.
- False negatives can occur in alkaline or dilute urine or when primary protein is not albumin. More accurate method is to precipitate urinary proteins with 3% sulfosalicylic acid (detects at 15mg/dl and detects other proteins). If urine negative on dipstick but strongly positive with sulfosalicylic acid, suspect multiple myeloma. With positive test use quantitative 24 hour urinary collection and test with protein electrophoresis or immunoassay.
- Transient; occurs commonly especially in children and usually resolves within a few days often associated with fever, exercise or stress. In older patients may be due to congestive heart failure.
- Intermittent; frequently associated with postural changes. Commonly occurs in upright position in young adults and rarely exceeds 1g/day. Resolves spontaneously in about half of patients and not associated with disease. If normal renal function evaluate no further.
- Persistent; usually due to glomerular cause with >2g protein/day of which major component is albumin. Some may also coexist with haematuria.
- Useful screen for diabetes mellitus.
- Urine testing for glucose is also useful in patients who find blood glucose monitoring difficult. Tests for glucose range from reagent strips specific for glucose to reagent tablets which detect all reducing sugars. Tests for ketones by patients are rarely required unless they become unwell.
- Nearly all glucose filtered by the glomeruli is reabsorbed in the proximal tubules and only undetectable amounts appear in urine in healthy patients. Above renal threshold (180mg/dl) glucose will appear in urine. Test relies upon reaction of glucose with glucose oxidase on dipstick to form hydrogen peroxide which causes colour change. This is specific to glucose and no other sugar.
- Occurs in diabetic ketoacidosis, pregnancy and following starvation or rapid weight loss.
- Dipstick test presence of aceoacetic acid at 5-10mg/dl but not acetone or beta-hydroxybutyric acid.
Bilirubin and urobilinogen
- Urine normally contains no bilirubin and only very little urobilinogen.
- Conjugated bilirubin only appears in urine in presence of liver disease or obstruction of bile duct.
Leucocyte esterase and nitrite test
- Leucocyte esterase activity is due to presence of white blood cells in urine.
- Nitrites strongly suggest bacteriuria. They are present because many species of gram-negative bacteria convert nitrates to nitrites.
- Test has variable sensitivity but high specificity. May not be a suitable substitute for microscopic examination.
Note a macro analysis of the urine sample is carried out in the practice and the sample is also sent to St James or St Vincents for culture and sensitivity.