Inadequacy of Dipstick Proteinuria in Hypertensive Pregnancy

Authors

  • M.A. Brown FRACP, MD,

    Corresponding author
    1. Departments of Renal Medicine and Medicine, St. George Hospital and University of New South Wales
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    • 1

      Associate Professor of Medicine, Renal Physician.

  • M.L. Buddie RN

    1. Departments of Renal Medicine and Medicine, St. George Hospital and University of New South Wales
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    • 2

      Hypertension Unit Nurse.


Department of Renal Medicine, St. George Hospital, Kogarah, NSW 2217, Australia.

Abstract

EDITORIAL COMMENT": It is absolutely essential with regard to patient management to know whether an antenatal patient with hypertension and/or generalized oedema has proteinuria. There is always the problem of contamination of urine and hence the need for a careful mid-stream specimen or a catheter specimen of urine rather than sending a patient home who has significant proteinuria and hence the risk of developing severe preeclampsia or eclampsia. A useful method to obtain a quick answer is to acidify the urine and boil in a test tube and allow to stand in a rack for 30 minutes when the amount of protein deposited can be assessed in terms of the whole volume of the specimen. When the protein is ‘solid'there is no visible urine above the precipitate while with ‘half-solid protein'the latter occupies one-half of the total urine depth. Qualitatively protein ‘half-solid'corresponds to 10–12 g/JL Although dipsticks are used widely in all antenatal clinics there is still a place for boiling urine to decide whether or not a patient requires admission to hospital as she certainly does when she has hypertension associated with significant proteinuria. After the woman has been admitted to hospital a 24-hour urine specimen should be collected to quantify the proteinuria.

Authors' Response to Editorial Comment: We agree entirely that one cannot overestimate the importance of detecting true proteinuria in hypertensive pregnant women. The thrust of our manuscript was to emphasize the inherent inaccuracies in dipstick testing when compared with 24-hour-urine protein quantitation as the gold standard. On the basis of this work alone we would not recommend routine use of boiling urine - to our knowledge no study has compared the relative accuracy of dipstick testing and boiling precipitation methods in pregnant women and it is widely recognized that both methods have their false negatives and false positives. Some authorities believe that “bedside ” precipitation techniques are outdated, stating that ‘old screening methods such as boiling the urine or detecting urine turbidity by eye after adding salicylsulphonic acid or trichloroacetic acid are much less reliable than the test strip’ (Mallich NP, Short CD. The clinical approach to haematuria and proteinuria, in: Cameron S, Davison AM, Grundfeld JP, Kerr D, Ritz E (eds). Oxford Textbook of Clinical Nephrology. Oxford Medical Publications. Oxford 1992: 227–228.

The message we wish to convey from this research is that once proteinuria has been ‘detected’ by a screening method (in this study dipstick testing) 24-hour-urine protein estimation is generally required to be certain about the significance of the screening test. Here we also agree with the Editorial Comment. Hopefully an accurate, reproducible and more convenient system of detecting proteinuria in pregnancy will be developed in the years to come.

Summary: The objective of this study was to determine the accuracy of ward urinalysis and the sensitivity of dipstick testing in the assessment of proteinuria in hypertensive pregnant women. Subjects were 230 consecutive hypertensive pregnant women who were admitted to hospital over a 2-year period. Routine ward urinalyses for protein, obtained on a mid-stream sample before and after a 24-hour urine collection for quantitating proteinuria, were compared with the 24-hour urine protein excretion. As a control for dipstick accuracy, urinalysis was also performed on a mixed aliquot of each of the 24-hour samples by a single observer experienced in urinalysis. True proteinuria was considered as > 300 mg/day.

The positive predictive value for urinalysis ranged from 38% (for the precollection test) to 60% (for tests on the aliquot). Negative predictive values were 86–88%.

The false negative rates at ‘nil’ or ‘trace’ proteinuria ranged from 8–18%. The false positive rates at 3+ (3 g/L) or 4+ (≥ 20 g/L) ranged from 0–17%, at 2+ (1 g/L) from 18–50% and at 1+(0.3 g/L) from 67–83%. Best results for urinalysis were obtained on the aliquot testing but even under these ideal circumstances there was a high false positive rate (67%) at 1+ (0.3 g/L) urinalysis level.

These studies show that in routine clinical practice ‘nil’ or ‘trace’ proteinuria will miss significant proteinuria in approximately 1 out of 8 hypertensive pregnant women while 3+(3 g/L) or 4+(520 g/L) will rarely be a false positive. At urinalysis of 1+ or 2+ a 24-hour urine collection is required to be certain about the presence or absence of proteinuria. Research studies should demand 24-hour urine protein quantitation and not rely solely upon urinalysis results.

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