An evaluation of a C-reactive protein assay using a rate immunonephelometric procedure

Biomarker-guided antibiotic therapy in adult critically ill patients: a critical review

Biomarkers of infection, namely C-reactive protein and procalcitonin (PCT), are potentially useful in the diagnosis of infection as well as in the assessment of its response to antibiotic therapy. C-reactive protein variations overtime appears to have a good performance for the diagnosis of infection. Procalcitonin shows a better correlation with clinical severity. In addition, to overcome the worldwide problem of antibiotic overuse as well as misuse, biomarker guidance of antibiotic stewardship represents a promising new approach. In several randomized, controlled trials, including adult critically ill patients, PCT guidance was repeatedly associated with a decrease in the duration of antibiotic therapy. However, these trials present several limitations, namely high rate of patients' exclusion, high rate of algorithm overruling, long duration of antibiotic therapy in the control group, disregard the effect of renal failure on PCT level, and above all a possible higher mortality and higher late organ failure in the PCT arm. In addition, some infections (e.g., endocarditis) as well as frequent nosocomial bacteria (e.g., Pseudomonas aeruginosa) are not suitable to be assessed by PCT algorithms. Therefore, the true value of PCT-guided algorithm of antibiotic stewardship in assisting the clinical decision-making process at the bedside remains uncertain. Future studies should take into account the issues identified in the present review.

The acute-phase reactant C-reactive protein binds to phosphorylcholine-expressing Neisseria meningitidis and increases uptake by human phagocytes

Neisseria meningitidis is a global cause of meningitis and septicemia. Immunity to N. meningitidis involves both innate and specific mechanisms with killing by serum bactericidal activity and phagocytic cells. C-reactive protein (CRP) is an acute-phase serum protein that has been shown to help protect the host from several bacterial pathogens, which it recognizes by binding to phosphorylcholine (PC) on their surfaces. Pathogenic Neisseria species can exhibit phase-variable PC modification on type 1 and 2 pili. We have shown that CRP can bind to piliated meningococci in a classical calcium-dependent manner. The binding of CRP to the meningococcus was concentration dependent, of low affinity, and specific for PC. CRP appears to act as an opsonin for N. meningitidis, as CRP-opsonized bacteria showed increased uptake by human macrophages and neutrophils. Further investigation into the downstream effects of CRP-bound N. meningitidis may lead us to a better understanding of meningococcal infection and help direct more effective therapeutic interventions.

Laboratory tests in critical care

A simple clinical index can be utilized to identify occult bacterial infections in the critical care unit. Use of this index enables the critical care physician to estimate the likelihood of occult infection, thus reducing and directing the diagnostic effort. This article reviews nonspecific screening tests used in the index.

Acute phase proteins in the monitoring of inflammatory disorders

The acute phase reaction is in most circumstances a good indicator of (local) inflammatory activity and tissue damage. CRP is a direct and quantitative measure for the acute phase reaction and due to its fast kinetics provides adequate information of the actual situation. The ESR on the contrary is in fact an indirect measure of the acute phase reaction. It does react much slower to changes of inflammatory activity and is influenced by a number of other factors. From studies on the 'behaviour' of CRP it has become clear that diseases may differ with regard to the extent in which they induce an acute phase response. Incidental measurement of the CRP level may add to the diagnostic procedure in selected cases, e.g. in the differentiation between a bacterial and a viral infection or between a bacterial infection and an exacerbation of diseases like SLE. In case of an extremely elevated CRP level (> 100 mg/litre) the possibility of a bacterial infection should always be considered. In clinical practice CRP is particularly useful when serial measurements are performed. The course of the CRP level may be useful for the monitoring of the effect of treatment and for the early detection of postoperative complications or intercurrent infections. The relationship between CRP and the local production and effects of cytokines on the one hand, and the possible functional role of CRP in the inflammatory process on the other hand have surely added a dimension to the clinical use of CRP as a parameter of inflammatory activity.

The usefulness of C-reactive protein levels in the identification of concurrent septic arthritis in children who have acute hematogenous osteomyelitis. A comparison with the usefulness of the erythrocyte sedimentation rate and the white blood-cell count

Thirty-six children who had bacteriologically confirmed acute hematogenous osteomyelitis but did not have concurrent septic arthritis, and ten children who had confirmed acute hematogenous osteomyelitis and concurrent septic arthritis, were followed for one year to compare the changes in the C-reactive protein level in the blood, the erythrocyte sedimentation rate, and the white blood-cell count. In both groups, the mean C-reactive-protein values were high (eighty-four milligrams per liter in the children who had septic arthritis and osteomyelitis and sixty-five milligrams per liter in those who had osteomyelitis only) at the time of admission to the hospital. However, in the group that had septic arthritis, the increase was significantly higher (p < 0.01) as early as the second day and a normal level (less than twenty milligrams per liter) was reached significantly later (p < 0.001) than in the group that had osteomyelitis only (11 +/- 7 days compared with 6 +/- 3 days [mean and standard deviation]). The erythrocyte sedimentation rate showed the same tendency, but the difference in the rates between the groups did not become evident until the fifth to fourteenth days after admission. A normal erythrocyte sedimentation rate (less than twenty millimeters per hour) was reached in 25 +/- 12 days in the children who had septic arthritis and in 17 +/- 10 days in those who did not (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Receiver operating characteristic curves for comparison of serial neutrophil band forms and C reactive protein in neonates at risk of infection

The performance of indirect indices of infection in the newborn vary because of differences in techniques, including diagnostic cut off levels. We have compared serial neutrophil band cell counts with C reactive protein measured by rate nephelometry. The 'gold standard' was a positive culture and the performance of the tests was compared by the technique of receiver operating characteristics (ROC) as well as sensitivity and specificity. A total of 172 septic screens were performed in 56 patients. The operational diagnostic cut off values were: C reactive protein greater than 8 mg/l, immature:total neutrophil ratio (I:T ratio) greater than 0.2, and band count greater than 5%. Compared with the sensitivity of C reactive protein (71%), I:T ratio (34%) was significantly different but band count (69%) was not. The specificity of C reactive protein (72%) was better than band count (39%) but no better than I:T ratio (73%). ROC curves were constructed for all possible diagnostic cut off values of the tests and superior performance was demonstrated for C reactive protein compared with band count and I:T ratio. We conclude that C reactive protein is a useful early indicator of infection in neonates and that ROC curves permit comprehensive and graphic comparison between tests and the calculation of optimal diagnostic cut off values.

Serum C-reactive protein and neopterin concentrations in patients with viral or bacterial infection

Serum C-reactive protein and neopterin concentrations were measured in samples taken at an early stage in different types of infection to see whether the combination of markers could contribute to the diagnosis of infection and help distinguish between bacterial and viral infections, tuberculosis, and infections due to "other" pathogens. Both markers were significantly raised in all categories of infection compared with controls, and there were significant differences between the means of both markers when comparing several of the categories of infection. Only C-reactive protein concentrations in bacteraemic patients, however, were both sensitive and specific at distinguishing the type of infection. The additional use of neopterin estimation played only a minor part in increasing the specificity of diagnosis in tuberculosis and in viral infections. On the basis of this study it was not considered worth the time and expense of performing neopterin assays in addition to C-reactive protein estimations to differentiate viral from bacterial infection.

C-reactive protein: a critical review

We have reviewed the literature to determine the value of C-reactive protein (CRP) measurements in the diagnosis and management of a wide range of conditions. CRP levels are of value in 6 clinical situations: (a) monitoring the response to antibiotic treatment in patients with known bacterial infections, (b) in obstetric patients with premature rupture of membranes, a rise in CRP can give early warning of intrauterine infections, (c) differentiation between active disease and infections in patients with systemic lupus and ulcerative colitis where the level of response to active disease has been previously established, (d) as a measure of disease activity and response to disease-modifying drugs in rheumatoid arthritis, (e) early detection of complications in postoperative patients, (f) in differentiating between infection and graft-versus-host-disease in bone marrow transplant patients. CRP levels have been used in an attempt to differentiate between bacterial and viral infections in various clinical situations, however the published literature does not support this role.

Acute phase reactants and risk of bacterial meningitis among febrile infants and children

STUDY OBJECTIVE

To test the hypothesis that quantitation of either C-reactive protein (CRP) or the total peripheral WBC count can improve clinical detection of underlying bacterial meningitis among young febrile children.

DESIGN

Cross-sectional survey of selected symptoms of central nervous system infection, signs of meningeal irritation and/or elevated intracranial pressure, levels of CRP in serum, and total peripheral WBC counts among unselected pediatric patients undergoing lumbar punctures for evaluation of acute febrile illnesses.

SETTING

Emergency department and acute care "walk-in" clinic of an urban, university-affiliated general hospital.

PARTICIPANTS

160 previously well, acutely febrile infants and children (median age, 6 months).

RESULTS

The prevalence of bacterial meningitis was 6%. Sensitivity of symptoms was 1.00 and specificity was 0.17. Sensitivity of signs was 0.70 and specificity was 0.81. Of the acute phase reactants, sensitivity of a CRP level of more than 1.0 mg/dL was 0.80, while that of a total peripheral WBC count of more than 15,000/mm3 was 0.40. The presence of signs and/or a CRP level of more than 1.0 mg/dL correctly identified all children with bacterial meningitis (sensitivity, 1.00). The absence of signs and a CRP level of 1.0 mg/dL or less correctly identified 71 of 150 children without bacterial meningitis (specificity, 0.47). Of 125 children without meningeal signs, the combination of symptoms and a CRP level of more than 1.0 mg/dL correctly identified all three children with bacterial meningitis (sensitivity, 1.00). The absence of these symptoms and/or a CRP level of 1.0 mg/dL or less correctly identified 80 of 122 children without bacterial meningitis (specificity, 0.66).

CONCLUSION

Quantitation of CRP but not the total peripheral WBC count can increase the sensitivity of physical examination findings and the specificity of symptoms for the diagnosis of bacterial meningitis. Measurement of CRP in serum is useful as an adjunct to history and physical examination for the detection of acute bacterial meningitis in the acutely febrile child.

The accuracy of clinical findings and laparoscopy in pelvic inflammatory disease

The accuracy of clinical diagnosis for pelvic inflammatory disease was determined in 95 women who presented with pelvic pain to primary care physicians and then were referred to gynecologists. Laparoscopy or laparotomy with endometrial biopsy and fimbrial minibiopsy revealed that prevalence of pelvic inflammatory was 46% (44/95) and positive and negative predictive values of gynecologists were 74% (23/31) and 67% (43/64) (p = 0.0002). If histopathologic diagnosis was the standard, clinical accuracies of the gynecologists were no better than chance (p = 0.43), suggesting an expectation bias for visual diagnosis. Laparoscopy had a sensitivity of 50% (12/24) and a specificity of 80% (40/50) for salpingitis if the standard was fimbrial histopathologic diagnosis (p = 0.01). These results support the routine use of laparoscopy, supplemented when negative by endometrial and fimbrial minibiopsy, to accurately diagnose pelvic inflammatory disease.

C-reactive protein is a marker for the diagnosis of adnexitis

The study reports the C-reactive protein (CRP) plasma concentrations in 115 women with a presumed diagnosis of acute adnexitis. In addition to CRP, blood sedimentation rate, white blood cell count and the body temperature were evaluated and compared with the clinical findings. Diagnosis was confirmed or excluded by laparoscopy (n = 69) or laparotomy (n = 9). Clinical examinations and conventional laboratory examinations were of limited value in the diagnosis of acute adnexitis. In contrast, CRP was a highly sensitive indicator of inflammatory pelvic disease. Furthermore, the CRP determination was superior in assessing the efficacy of an antibiotic treatment.

Serum C-reactive protein in the early diagnosis of neonatal septicemia and bacterial meningitis

The usefulness of serum C-reactive protein (CRP) in the early detection of neonatal infection was studied using a special laser nephelometric apparatus (CRP-1), by which CRP concentrations could be quickly determined in the nursery, with only a small amount of serum (20 microL). Initial serum CRP concentrations of samples obtained from 90 infants suspected to have sepsis and/or meningitis were evaluated. Of the 90 infants, 25 showed culture-proven septicemia and/or bacterial meningitis, while 18 were considered to be infectious based on clinical signs and positive sepsis work-up even though cultures were negative. 47 infants had negative cultures and sepsis work-up and showed a favorable clinical courses. Statistical analysis for the evaluation of serum CRP at the level of one mg/dL was performed. False negative CRP was demonstrated in seven of 25 infants with culture-proven sepsis and/or meningitis (28%) and in 4 of 18 infants with other infections (22%). On the other hand, seven of 47 (15%) non-infected infants showed false positive results. The specificity and sensitivity of serum CRP determination were 85% and 74%, respectively, for all patients, and 85% and 72%, respectively, for patients with sepsis and/or meningitis. The sensitivity varied with the pathogens. We conclude that, while the initial CRP values alone are unsatisfactory for deciding the need for antibiotic therapy, CRP is useful in the early detection of neonatal infections, and its measurement by this new equipment should available in the nursery.

A two-site immunoradiometric assay for C-reactive protein in serum

An immunoradiometric assay was developed for C-reactive protein in serum. The assay had a sensitivity of 5 micrograms/l and good precision. Correlation with radial immunodiffusion (r = 0.916) and EMIT (r = 0.935) was close. A reference range for healthy adults of 0.05-4.0 mg/l was derived.

Diagnostic audit of C-reactive protein in neonatal infection

A prospective study of 250 consecutive neonatal admissions to a regional perinatal referral centre and of 10 additional consecutive cases with culture-proven neonatal septicaemia was undertaken. Quantitative C-reactive protein (CRP) determination, white cell count and differential were performed on blood samples obtained from all babies on admission, as well as 10-14 h and 22-26 h later. Using clinical signs, chest X-rays, blood cultures, tracheal aspirates obtained within 4 h of delivery and an abnormal immature/total neutrophil ratio (I/T), infected babies were defined as belonging to one of the following groups: culture-proven septicaemia (n = 19); clinical septicaemia (n = 35); congenital pneumonia (n = 28). The sensitivity, specificity, positive and negative predictive value of CRP were calculated for each sampling time and patient group. No baby had a rise in CRP (greater than 6 mg/l) before an abnormal I/T ratio was first detected. A delayed rise in CRP concentration in the majority of infected babies occurred approximately 12-24 h after the abnormal I/T ratio was first detected. The overall specificity of a CRP level of greater than or equal to 10 mg/l remained approximately constant (97%-94%) while sensitivity increased from 22%-61% with increasing time after admission. The same pattern emerged if each patient group was considered separately. The positive predictive value for a CRP level of greater than or equal to 10 mg/l 22-26 h after admission was 83% and the negative predictive value 82%. CRP had no value in the early diagnosis of neonatal infection. Its main role lies rather in the exclusion or confirmation of infection 24 h after the first clinical suspicion.

Evaluation of a semiquantitative C-reactive protein latex slide test as compared to quantitative turbidimetric measurement in hospital laboratory practice

Serum C-reactive protein (CRP) concentrations of 708 patients with suspected bacterial disease were estimated with a semiquantitative CRP latex slide test using cut-off levels of 20 and 40 mg/l and with quantitative turbidimetric technique. Latex slide tests gave 11.8% (80/708) discordant results. About 7% of tests gave clear disconcordant results (5% of those were positive and 2% negative tests). We conclude that CRP latex slide test is not useful in emergency laboratory in hospital material with a high incidence of bacterial infections.

Interference of IgM rheumatoid factor with nephelometric C-reactive protein determinations

Different nephelometric assay systems for quantitation of C-reactive protein (CRP) were compared with radial immunodiffusion (RID) and tested for their susceptibility to interference by serum IgM rheumatoid factor (RF). In 3 nephelometric assays, RF was found to elevate CRP values. Sera with high RF content from patients with rheumatoid arthritis gave significantly higher CRP values by nephelometric assay than by RID; the addition of purified RF to RF-negative sera increased CRP values markedly; and removal of RF from sera corrected falsely elevated CRP values. This interference by RF is explained by the action of human RF as a (secondary) antibody reacting with complexed mammalian IgG anti-human CRP in the assay. In this way the nephelometric signal is enhanced to give falsely elevated CRP values. In contrast, the gel diffusion RID method does not suffer from this non-specific interference.

C-reactive protein for detection and follow-up of bacterial and fungal infections in severely neutropenic patients with acute leukaemia

To evaluate the aetiology of febrile episodes and to rationalize our politics with antibiotics, C-reactive protein (CRP) was determined immunoturbidimetrically in 20 consecutive neutropenic adults with acute leukemia. They had 35 febrile episodes, 89% of which were infectious. Twenty per cent of infections were fungal. A similar CRP response was seen both in bacterial and in fungal infections. In 84% of infections the peak value for CRP rose greater than 100 mg/l. Thirty-five apyrexial patients with acute leukaemic and 20 healthy adults served as controls. Their CRP was less than 10 mg/l in 87%. CRP proved most valuable in the follow-up of infections, in the detection of infectious complications and in the detection of possible invasive fungal infections. Although relapse itself did not effect on CRP levels, extramedullary bone infiltration in two of our patients resulted in increased CRP production, which normalized with cytostatics only.

Sequential study of C reactive protein in neonatal septicaemia using a latex agglutination test

The usefulness of serial study of C reactive protein in the early detection of neonatal septicaemia was evaluated in a neonatal unit using a commercially available latex agglutination slide test as a rapid screening method and electroimmunoassay as a reference method for C reactive protein determination. A positive latex test was obtained in 11 infants with verified septicaemia (positive blood culture), two infants with clinically evident infection but without bacteriological confirmation, one infant with recurrent chest infection due to Pseudomonas aeruginosa, and one infant who showed signs of birth asphyxia with meconium aspiration, but was not infected. Positive latex test results correlated with raised concentrations of C reactive protein, measured by immunoassay. In some instances, however, concentrations of C reactive protein in excess of 12 mg/100 ml gave weaker agglutination results in the slide test, which could be interpreted as negative results. In a sequential study of the infected infants, 6.3% of the values recorded on a slide test were false negatives. In contrast, false positive values were observed on a slide test in 1.9% of 27 non-infected infants. The higher percentage of false negative values may be due to the presence of excess antigen in the sera of some infected children. It is suggested that the latex test should be carried out on suitable dilutions of serum. Although the slide test may reliably indicate infection at an early stage in neonates, the C reactive protein response is non-specific, as seen in a non-infected infant who showed signs of birth asphyxia with meconium aspiration. Provided the non-specific nature of the C reactive protein response is recognised, the latex test may be a useful serum measurement for early diagnosis of neonatal septicaemia of the newborn. The test has the advantage of being performed easily, quickly, and cheaply.

C-reactive protein in patients with bacteremia

Quantitative measurement of C-reactive protein (CRP) in serum has been proposed as a sensitive and, for some populations, a specific indicator of infection. To determine whether early measurement of CRP in serum could differentiate patients with bacteremia from a control group of patients whose blood cultures yielded contaminants, we measured CRP concentrations quantitatively by rate nephelometry in serum samples that had been obtained from patients on the same day as blood samples that yielded bacteria or fungi. Of the 36 episodes of bacteremia, 3 (8.5%) occurred in patients with normal concentrations of CRP in serum and 2 (5.5%) in patients with minimally elevated levels. Of the 21 episodes associated with contaminated blood cultures, only 2 (9.5%) occurred in patients with normal CRP levels. Of the patients with marked elevations of CRP (greater than 10 mg/dl), 18 (86%) had infection, although not all of these patients had bacteremia. We conclude that a normal concentration of CRP in serum does not eliminate the possibility of bacteremia. Moderate elevations (1 to 10 mg/dl) of CRP levels are common in both patients with contaminated blood cultures and in those with bacteremia. If the CRP concentration in serum is greater than 10 mg/dl and if other causes of marked elevations of CRP levels are eliminated, CRP concentration in serum may be a relatively specific indicator of infection. However, elevations of CRP concentrations are neither completely sensitive nor specific for detecting infection in patients with bacteremia.

C-reactive protein as a detector of organic complications during recovery from childhood purulent meningitis

Twenty-eight bacteriologically proved episodes of purulent meningitis in 27 infants and children were monitored prospectively with sequential determinations of serum C-reactive protein. Except in one rapidly fatal case, all the patients showed decreasing CRP values for about 1 week. In five patients the CRP values than returned to a high level (P less than 0.001). Each of these patients developed an organic complication (subdural effusions in three, transient widening of the ventricles in one, purulent arthritis with osteomyelitis in one). Except for one infant with sensorineural hearing loss, which probably had developed early in the course of meningitis, no permanent sequelae were found in the patients with uncomplicated courses. One infant later had a relapse of Escherichia coli meningitis, reflected in a sharp increase of CRP. We conclude that sequential CRP measurements may be performed routinely to detect potential complications at an early stage of bacterial meningitis.

Serial study of C reactive protein in neonatal septicaemia

Serial C reactive protein concentrations were assayed by electroimmunoassay in 41 infants. Values in most of the non-infected infants were below 0.3 mg/dl, the lower limit of detection of C reactive protein by electroimmunoassay. Eleven of 12 infants with proved sepsis (positive blood cultures) had significantly raised concentrations and one infant with recurrent pseudomonas chest infection had a raised C reactive protein concentration. High C reactive protein concentrations were also found in infants with suspected infection. Successful treatment was followed by a decrease in the C reactive protein concentration. Total white blood cell count was not as appropriate as C reactive protein determination in the early identification of bacterial infection in the newborn.

Measurement of murine serum amyloid P component by rate nephelometry

Murine serum amyloid P component (SAP) is an acute-phase protein that is increased 2-10-fold in concentration following appropriate inflammatory or infectious stimuli. Previous studies of the acute-phase SAP response have employed quantitative immunoelectrophoresis or radioimmunoassay to measure SAP concentration. A rate nephelometric procedure has been developed which measures SAP concentration rapidly and with equivalent or greater precision than the previously applied techniques. This simple method will facilitate experimental and clinical studies of the acute-phase response.

C-reactive protein concentration as a guide to antibiotic therapy in acute leukaemia

Serial monitoring of the serum C-reactive protein (CRP) concentration was performed, using a one-hour laser-immunonephelometric assay, during 29 episodes of infection in 22 neutropenic patients with acute leukaemia. Serum CRP increased to above a diagnostic level of 100 mg/l in all 29 episodes and continued to rise progressively until appropriate antibiotics, or granulocytes, were given when it fell with a half-life of approximately three days. Serial study of the serum concentration was of value in detecting occult or unresolved bacterial infection and provided an objective means of monitoring the response to antibiotic and granulocyte therapy.

C-reactive protein for rapid monitoring of infections of the central nervous system

Serum C-reactive protein (CRP) was measured nephelometrically or turbidimetrically for rapid differential diagnosis of sixteen bacterial and fifteen viral infections of the central nervous system in patients aged from 2 weeks to 49 years. On hospital admission CRP levels were far above the upper limit of normal (19 mg/l) in all patients with bacterial meningitis, regardless of the duration of illness, the age of the patient, the bacterium involved, fever, the erythrocyte sedimentation rate, or the cerebrospinal-fluid cell count. In contrast, a slight rise in CRP level was seen in only one case of viral meningitis. CRP was useful also in monitoring the clinical course of the illnesses and in the detection of subdural effusion in one patient with Haemophilus influenzae meningitis and of otitis media in another patient with coxsackie B meningitis. If no complications developed, CRP levels returned to normal within 7 days in the bacterial meningitis group. The rapid measurement of CRP levels is of importance and should be used more often in clinical practice.

Solid-phase immunoradiometric assay for C-reactive protein using magnetisable cellulose particles

An immunoradiometric assay (IRMA) for C-reactive protein (CRP) was developed using magnetisable cellulose particles as the solid-phase support for anti-CRP antibodies. 125I-labeled immunopurified anti-CRP antibody was used to quantitate the amount of CRP taken up by the solid phase. Unbound label was easily and rapidly removed by decantation after sedimenting the particles on a magnet. The assay could detect 1 microgram CRP/l and had a range of up to 10 mg/l with the portion of the standard curve between 10 micrograms/l and 2-3 mg/l being linear. Fifty samples per hour could be processed manually from serum to CRP result with an intra-assay CV of 5.2% and an inter-assay CV of 10.0%, based on 5 replicates of 5 samples with CRP levels between 2 mg/l and 180 mg/l run in 5 separate assays. Fifty clinical samples were assayed in parallel with a standard electroimmunoassay and yielded a linear correlation coefficient (r) of 0.975 and a slope of 0.988. With its single, brief incubation step including all reagents and its simple phase separation procedure the present method may be the assay of choice when precise measurement of CRP concentrations is required rapidly.

Immunonephelometric and immunoturbidimetric assays for proteins

Immunonephelometric and immunoturbidimetric techniques for the measurement of proteins have developed and expanded rapidly in recent years and are fast replacing the time-honoured gel precipitation techniques. The reasons for this are primarily an increased awareness of the value of specific protein measurements with the impetus of improved chemical reagents and advances in instrument technology. This review discusses the background to such free fluid phase immunochemical measurement systems and the developments which have occurred in this field. The following topics are reviewed. (1) The theoretical background of light scattering theory as applied to the measurement of antibody-antigen complexes. (2) The nature and kinetics of the antibody-antigen reaction in fluid media and the effect of enhancing polymers. (3) The sample and antibody requirements for nephelometric and turbidimetric assay. (4) Instrumental systems for nephelometry. (5) Instrumental systems for turbidimetry. (6) Methods of establishing, assessing and monitoring nephelometric assays for specific proteins in the laboratory.