CONTINUING MEDICAL EDUCATION
CASE REPORT

 

The utility of urine sulphosalicylic acid testing in the detection of non-albumin proteinuria

 

 

S Ndamase^I; R Freercks^II

^IMB ChB; Division of Nephrology and Hypertension, Department of Medicine, Faculty of Health Sciences, University of Cape Town, and Renal Unit, Livingstone Hospital, Port Elizabeth, South Africa
^IIMB ChB, FCP (SA), Cert Nephrology (SA), MPhil; Division of Nephrology and Hypertension, Department of Medicine, Faculty of Health Sciences, University of Cape Town, and Renal Unit, Livingstone Hospital, Port Elizabeth, South Africa

Correspondence

 

 

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ABSTRACT

We report two cases of immunoglobulin light chain proteinuria (Bence Jones proteinuria) detected by simple side-room investigations: urine dipstick negative/1+, but with strong positive precipitation on addition of an equal volume of sulphosalicylic acid (SSA) 3%. We highlight a significant limitation of urine dipstick testing, namely specificity for albumin, and the utility of SSA testing for the detection of urinary free light chain immunoglobulins.

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Case 1

A 51-year-old woman with hypertension and type 2 diabetes mellitus was referred for evaluation of her chronic kidney disease. Her creatinine was 124 μηιοΙ/L, urea 5.6 mmol/L and estimated glomerular filtration rate 40 mL/min. She had a severe normocytic anaemia, with a haemoglobin of 6.5 g/dL. On further enquiry she gave a history of unintentional weight loss of 9 kg. She was pale, with no lymphadenopathy or oedema. Urine dipstick was negative for proteinuria, but the urine protein:creatinine ratio was 0.8 g/mmol (normal range <0.02 g/mmol), indicating nephrotic range proteinuria. The addition of sulphosalicylic acid (SSA) to her urine caused immediate turbidity (Fig. 1). A diagnosis of multiple myeloma was therefore suspected and serum and urine electrophoresis confirmed a monoclonal lambda peak. Serum free kappa light chains were 12.9 mg/L (3.3 - 19.4), and free lambda light chains 5 688 mg/L (5.7 - 26.3).

 

 

Case 2

A 54-year-old man presented with a 2-day history of headache and epistaxis. He was pale, with no lymphadenopathy and an otherwise normal physical examination. Initial testing revealed a severe bicytopenia: haemoglobin 6.2 g/dL, mean corpuscular volume 85.8 fL, and platelets 10 x 10⁹/L. A peripheral smear showed occasional teardrop cells with a leuco-erythroblastic reaction and no fragments or platelet clumping. His urea was 14.9 mmol/L and creatinine 460 μmol/L. Urinary dipstick testing showed only 1+ proteinuria, while the urine protein:creatinine ratio was markedly elevated at 1.3 g/mmol. On addition of SSA, dense turbidity was immediately noted. A diagnosis of multiple myeloma was subsequently confirmed on bone marrow biopsy. Serum free kappa light chains were 8.2 mg/L (3.3 - 19.4) and lambda light chains 27 098.0 mg/L (5.5 - 26.3).

 

Discussion

The urine dipstick is specific for albumin and will miss positively charged proteins in the urine, such as immunoglobulin light chains.^[1]The bedside detection of non-albumin proteinuria is aided by the use of a simple, inexpensive and often-overlooked investigation: addition of SSA 3% to urine. The degree of turbidity provides a semiquantitative method for proteinuria detection.^[2] Discordance between SSA and dipstick testing (where SSA is strongly positive and dipstick negative or low positive) mainly suggests the presence of non-albumin proteinuria, usually immunoglobulin light chain excretion.^[3] This reminder may be of particular interest to clinicians in the South African public sector, where serum and urine electrophoresis results can take 3 - 4 weeks to become available. However, SSA should not substitute urine electrophoresis, as small quantities of monoclonal free light chains may be missed^[4]

 

References

1. Kashif W, Siddiqi N, Dincer HE, et al Proteinuria: How to evaluate an important finding. Cleve Clin J Med 2003;70(6):535-547. [http://dx.doi.org/10.3949/ccjm.70.6.535]        [ Links ]

2. Arnulfo J, Penagos V, Jairo J, et al. Use of sulfosalicylic acid in the detection of proteinuria and its application to hypertensive problems in pregnancy. Iatreia 2011;24(3):259-266.         [ Links ]

3. Gyure W. Comparison of several methods for semiquantitative determination of urinary protein. Clin Chem 1977;23(5):876-879.         [ Links ]

4. Said A, Naderi A, Reilly RF. Primary care approach to proteinuria. J Am Board Fam Med 2008;21(6):569-574. [http://dx.doi.org/10.3122/jabfm.2008.06.070080]        [ Links ]

 

 

Correspondence:
R Freercks
robert.freercks@uct.ac.za